Crushing is a concept that indicated invincibility to a specific type or types of attacks. Crushing moves can ignore by a level of attack: highs, mids, special mids, lows and/or by a type: attack moves, throws, projectiles.
Crushing is often confused with evasion.
Crushing moves are moves which ignore collision between the character's hurtboxes(of the player getting hit) and hitboxes(of the attacking player) of types of moves: even if hurtboxes collide with hitboxes, this fact is ignored by the game because of crushing properties of the move. Evasion happens because of lack of collision between character's hurtboxes and move's hitboxes: the game will register a hit if hurtboxes and hitboxes will collide and evasion happens when they don't.
Prime example would be hopkicks for crushing lows and Paul's backsway for evasion. Hopkicks are known to crush lows because they are invincible to lows due to the airborne state it gives to the player(starting at n-th frame), while Paul's backsway causes him to evade moves, which creates whiffs leading to a whiff punish.