Can someone please explain

brighter = sharper-sounding midrange and/or treble and tight bass
darker = like brighter above, but more accurate
warmer = softer-sounding midrange/treble and more bass

Neutral sound means more accurate. Ideally, you don’t want to boost any particular frequency. The music should come together as one cohesive whole and no frequency should annoyingly stand out.

You can learn by looking at the waveforms of the track you’re listening to. Try to make some EQ changes using something like Wavepad and see where that goes.

Some tracks sound like "recordings" while others sound like organic music. It all depends on many factors...but the recording quality itself is of the utmost importance (using the right microphones).

Look up amplifier classes to learn more. SS or tubes? that's a personal preference. Good luck.

All of these terms are just "poetic license"! I'd rather look at frequency and pulse response, polar dispersion and power handling. Speakers that measure better will sound better. Unlike electronics speakers haven't evolved  to the point where their flaws are hidden from measurements!

These terms are listener oriented audio terms. While waveforms / theories are interesting if you are an engineer they are not of much actual value to a audio enthusiast. These terms and others are important in comparing musical output from a system or component.

This may be a bit overwhelming but here is a glossary from Stereophile.

https://www.stereophile.com/content/sounds-audio-glossary-glossary

I recommend reading Stereophile And The Absolute Sound for reviews to get a basic understanding. Specific questions can be answered on the forums... but you can get a lot of “religious zealots” and biased opinions, some well meaning, some not. While these publications have specifications associated with equipment the bulk of the content and value is about listening comparisons.
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Inexpensive equipment frequently is bright... with a high preponderance of treble, which over time you may realize is actually distortion. So, bright may be very detailed sounding... and you may find it fatiguing. Dark can mean an absence of treble.
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But they are relative terms. So you need a ruler against which to judge. If you have equipment find every review and comment on line about it so you can understand how it is described. If you do not, or it is too low end. Go to a high end audio store. Tell them what kind of music you like and find some equipment you like. Write it down. Then read all about it from sources like above or Tone Magazine, positive feedback. Try and get to know a well reviewed brand. Audio Research, MacIntosh, B&W, NAD,.. if your find this kind of research interesting, this may be a good hobby for you. It is very complex, and specifications are useful for really gross decisions on the sound you want. This is about listening and talking about overall tonal balance, slam, sound stage, and rhythm & pace... the musicality... does it emotionally connect with you. 
A good manor to learn and acquire is to find some flagship products who’s sound you really like and the work down their product ines for something you can afford. Most high-end companies have a house sound which applies to all products. So you can move up their product lines over time.

GOOD LUCK.

How do you not know it’s actually in the recording?

In my case I was there when the recording was made.

What you mean when you say (whether it’s a speaker or amp or preamp) it’s darker or warmer or softer or leans to the brighter side of neutral?

Most of audiophile conversation in this regard is referring to distortion. Its distortion that causes solid state amps to sound bright (which is to say it sounds a bit like when a treble control is turned up; turning down a treble control causes things to sound 'darker') and harsh. 'Warm' is often caused by the 2nd and 3rd harmonics and is more often associated with tube amplifiers, the 2nd in particular.


Both are colorations. Because brightness and harshness is annoying and fatiguing, its arguable that erring towards lower ordered harmonics (which are less annoying and to which the ear is far less sensitive) could be construed as 'more neutral'. But you have good reason to be suspicious; if you were not in the vicinity when the recording was made there really isn't any way to know what is right. To get around that you simply have to play a lot of records and on a lot of systems :) If you attend live concerts and know what the instruments sound like, that is really helpful.

"...Speakers that measure better will sound better..."

I'm going to guess you are not a Magnepan owner. 

Curious minds/ears want to know.
Yeah and just asked a book full of questions. Specifically, The Complete Guide to High End Audio by Robert Harley, which you definitely need to drop everything and buy right now. 

Meantime here are a few brief answers to help you appreciate just how deep these waters are. 

What you mean when you say (whether it’s a speaker or amp or preamp) it’s darker or warmer or softer or leans to the brighter side of neutral?

Wow only SIX completely different things all lumped together as if they are one! 

A lot of audio terms are visual metaphors. Imagine you are in a room and looking around and there is just enough light to see everything, except for maybe in a few corners here and there. Replace corners with musical details and this is dark. So you turn up the lights and now you can see into the darkest corners but guess what? Now it is maybe a little too bright. Hurts your eyes. Or ears. These are all metaphors, remember?

Warmer is you go to a warm bulb instead of the daylight. The color balance shifts. Replace color with tone and now you got a handle on warm. 

Neutral is a fantasy. Do you have any idea what neutral lighting is? All neutral means is whoever is using it, this is his idea of whatever is the midpoint between bright and dark, warm and analytical, etc. Neutral is no more objective than anything else. If you can just manage to learn this one thing you will be miles ahead of the game.

Are you talking about how ss compares to tubes to class D?

Now you are getting even more confusingly generalized. All these things can really only be intelligently compared in terms of how they actually sound. When you have done this a very, very long time you may be able to spot certain things by their sound signature alone. I heard one bit of silicon in a tube phono stage without knowing, just heard it. But you think it was fun writing all this so far? What I would have to write to explain just that one thing would be twice all this put together. 

Analog to digital? How do you know what "neutral" is? How do you not know it’s actually in the recording?

More of the same. The really good one is how do you not know it's actually in the recording. That is the million dollar question. Right now you have no freaking clue. Here's the good part: right now neither do I! Listen to records, you start to think you know what is in the recording- until you get a really good copy of the same record and then realize what you were hearing was a pressing, not the actual recording. Now you are much closer to the actual recording, but still not there!

The really crazy part, it does no good to say you were there when the recording was made. All that means is you know what the instruments sounded like to you when you were in the room. That is not what they sounded like to the microphone you used in the room. You know what it sounded like when it was mixed down in the recording studio. Big deal! Are you saying we all have to go back in time to the recording studio now? Most of those are set up for what they are supposed to do, which is completely different than what we are doing, relaxing at home or whatever. Not sitting in front of a monster console looking through plate glass at a sound room.

Read the book. Learn the jargon. Master all the many subjects. Eventually you will be ready to come back to where you are right now: does it sound good? Can you afford it? Well, all righty then....

If you attend live concerts and know what the instruments sound like, that is really helpful.

i do not know if that is neutral but is a very good starting point.

How do you not know it’s actually in the recording?

more sound is less noise

G

when someone makes these statements about gear it has obvious tonal issues that stand out and that is why these adjectives are used neutral is when it sounds right to your ears on your speakers in your room when you find a neutral amp in your setup other amplifiers will be bright/dark, warmer/brighter, etc.

How do you not know it’s actually in the recording?

If everything you play has the same type of sound, you know it's not in the recording. Recordings are individual things and they will not all have the same type of sound. Some artists, producers, labels, etc., may have a house sound. However it would be statistically unlikely that every different recording you listen to has the same anomaly.

brighter = shrill sounding, no bass, weak midrange, metal dome bad tweeter. NHT with bryston for example.
warmer = relaxed treble, good midrange, clean bass
darker = mixing a warm sounding amp with a warm sounding speaker. Sonus Faber speakers with a tube or Mcintosh/Classe amp for example.
Neutral - a speaker that does pretty much everything right
Goal is to get either a warmer or neutral sounding speaker and compliment it with an appropriate front end

Compare the sound of a speaker with the sound of live music.
What are the differences?
Then all these descriptors snap into focus:
The speaker is 'brighter' than the performance.
Or 'warmer'
etc

well, ears are like mussels need to be introduced to physical activity, developed and trained:))), but some capabilities are congenital 

ears look more like shrimp than mussels ...

the shrimp is actually big mussel itself:)))

Some mussels are congenital. Just ask Arnold's maid.

@millercarbon 
some mussels definitely are, but their function is not always congenital... the Musculus Cremaster is good example :))) 

Takes balls to even mention that one. 

A great example how an honest and  innocent question, in this case of sound quality can turn. From soup to nuts ....

@isochronism 
Don’t pay attention to distracting little details. The main point is congenital factor of sound perception.  

When I look at a painting I generally view it as a whole creation.....not individual components of paint colors/textures, tints, composition, view angle, etc. (abstract stuff too).
With music, for me, the same thing goes....and I either actively engage with it or don’t.
Hear the music and decide on a macro level or drive yourself crazy.
I’ll never be unequivocally sure what the artist intended and/or what happened in the daisy-chain between the creation and my ears.....but I do know if I like it or not.
I’ll probably be banned for saying this, but please try to ignore what you MAY be missing (really or not) and just enjoy the pleasure of experiencing the music with what equipment you’ve got.

please try to ignore what you MAY be missing

silly point of view. Ignoring the fact of subject existing does not cancel the fact. The person who just trust will never click  with person who try to understand.

When you listen to your playing music, you (rather obviously) are hearing everything working together - the sum total of what your equipment does to the the sound of the recording being played. One earlier commenter was completely correct: we are all at the mercy of the recording engineer. After that, we have some control. Acoustic instruments are a good guide to accuracy. It's pretty easy to hear guitars, ukuleles, banjos, mandolins, and the whole violin family played in a place near you. Go to amateur events, music club meetings, school presentations and such near you. Listen to the sound of the instruments, however well played (or not). Compare the sound of a banjo with a steel rim verses an aluminum one (aluminum is brighter as well as less heavy and cheaper, steel is darker, heavier, more expensive). How does a big bodied guitar sound in comparison to a smaller, more hourglass shaped instrument? Now play music recordings of those instruments and see how that compares to your memory of the live instruments. All of us have differently shaped ears. No two of us hear exactly the same thing, so the definition of "really good" equipment is always subjective. Don't be too concerned with someone else's opinion. Start with well reviewed (that does NOT mean expensive) equipment, perhaps bought "used" - read: save a bundle - and speakers that seem good to you. Speakers make the most difference in what you'll hear. The room you are listening in is the next most important factor. That's why headphones are so popular. Wires, fancy or otherwise, make the least difference. Many professional musicians never listen to recorded music because the sound is foreign to them. They put their money into good instruments. When you have a system sounds good to you, enjoy it and don't invite any critical people over to your house! Keep Smiling! Enjoy the music!