What is "break in" and what difference does it make? In amps? Preamps? Speakers? More?

Sorry to ask a question that’s been asked so many times. If it were possible, I’d just delete this question and try to come up with something more novel. But I suppose people can just skip answering it. Or give 

Thanks for your answer. My general comment about what folks have said so far is that I understand (of course) the concept of what "breaking in" means -- and the phenomenological characteristics associated with shoes, shirts, even the way a lawn mower starts right up (or a key fits a lock) are all familiar. What is helpful in these posts (and in the many other posts which I will go check out), is when some specifics about what-one-would-experience with a broken-in-preamp vs. a broken-in-amp vs. a broken-in-speaker, etc.

Your answer, Millercarbon, makes sense to me -- "All start out with whatever fundamental character they have, but thin and etched or grainy, and then over time fill in and round out becoming more palpably real." That is a recipe for simply looking for what's dominant in the character of the thing and then watching as it deepens/enhances. Beyond that, what I wonder about is whether or not, for example, one can count on certain *missing* or *faint* characteristics (e.g. "lack of defined midrange detail in a speaker") to emerge or grow after "break in." That such a think could happen (and when, where, how) could conceivably help someone decide to wait on a speaker, amp, etc. before trading it in, prematurely. 

Thanks, all. That definitely helps.

@asvjerry I'm getting good answers here and realizing a thick skin may be necessary. I'm a college professor, of philosophy. I am not at a highly selective school and many are first generation college students. Every semester faculty at my school are asked very similar questions by new students. We find that even answering questions we've had before teaches us something, so long as we try to appreciate the perspective and practical motivations of the student. It is professional suicide to blame the learner. 

I hear you. Thanks! I guess I was also wanting to try a little engagement with folks in the group. There are very few people I know who are into audio, it's good to try a few interactions.