Guidelines behind the scenes, invisible to the player, that determine the space a fighter occupies, and the places where they’re vulnerable. Hurtboxes for both taking damage and physically pushing against opponents pretty much conform to what you see the fighters doing, with one quirk: the really big, tall fighters (mainly Charizard, Ganondorf, Incineroar, and Ridley) have big vertical hitboxes for absorbing incoming attacks, but other fighters who hop against their towering upper bodies simply pass through. As a concession to smooth gameplay, the huge dudes aren’t physical impediments to smaller fighters leaping through short-hopping airspace.
Contrast with hitboxes, the hitting portions of moves.