Blackness - how quiet does it need to be?

In almost all gear of any substantial value the concept of the blackness, quietness or low noise floor comes up. A reviewer might say that the noise floor was noticably lower when reviewing a particular piece compared to another. Now I get that low noise translates roughly to being able to hear more music and nuanced detail. Thing is, when I turn on my system and no music is going through it, I can't hear anything, unless I put my ear right up to the speaker and the AC isn't running and the fan isn't on, etc. And with music on the only thing I hear is any recorded hiss that might be from the recording. So what I dont get is when they say a piece of equipment sounds quieter, do they mean somehow that the hiss on the recording is lower? I cant see how that would be possible, or are they talking about the hiss of the equipment without muisc? In which case I cant hear it at all when sitting down on my couch. I don't have the world best gear, so I'm thinking are they overplaying the "quiet" card.