Crushing is a concept that indicated invincibility to a specific type or types of attacks. Crushing moves can ignore by the level of attack: highs, mids, special mids, lows or by type: attack moves, throws, projectiles.
Crushing often confused with evasion. Crushing moves are moves which ignore collision between the character's hurtboxes and hitboxes of types of moves: even if hitboxes collide with hitboxes, they're ignored by the game because of crushing. Evasion happens because of lack of collision between character's hurtboxes and move's hitboxes: the game will register a hit if these 2 will collide. 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, while Paul's backsway causes him to evade highs which creates whiffs leading to a whiff punish.