IME, new guitars, even very good new guitars, are a tiny bit bright and may lack a bit of warmth or depth. Think of the top as an acoustic panel that has to vibrate to make sound - it is in effect a speaker cone. That top starts off stiff and over time starts to loosen up, as the fibres break down. Springs do the same thing, elastic, all sorts of materials. Audio speakers too, but in that case it's the spider and surround as the cone is supposed to be stiff and move as a piston.
So as guitar tops, and the other body woods to a lesser extent, vibrate and flex, they break down a bit and loosen up, or 'open up', as people say. The sound gets less bright and a bit warmer (midrange and bass get a little fuller). The change is subtle and happens over months or years and not days or weeks.
There is a debate on whether artificially aged (torrefied) woods will open up any more. I don't expect the very old mahogany on my Lowden to change much, but the Adi top (it's only a decade old) will open up a bit over the next year or two. It sounds pretty amazing right now, so I can only imagine it getting better.