We’ve seen it many, many times: the ship gets into a firefight, takes a few hits, shakes around, and consoles explode (possibly taking an unfortunate ensign with them). Eventually the battle is resolved with our heroes largely intact if somewhat shaken up. If it was a particularly nasty battle, there will be signs of damage: scorches on the walls, deformed equipment, busted lights, and rocks scattered about.
All of that seems reasonable… except the rocks, which look pretty out of place in a spiffy 24th century starship. So why are they there?
One common fan theory is that it’s slag from melted components, or debris from things like damaged screens/lights, which is why it seems to be a constant throughout most Starfleet ships.