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

Hi folks,

Newbie question. I read often about a break-in period for speakers, amps. Can someone explain what this means, technically and to the listener's ears?

Is there a difference in what one hears when it comes to speaker break-in vs. component break-in?

Are there levels (quality) where break-in makes no difference?

Thanks.