A monstrous creature is so huge and powerful that its attacks make a mockery of armour, foes are not so much wounded as hideously mangled and crushed.
A monstrous model (Carnifex, Greater Daemon etc,) ignores armour saves.
Monstrous Creatures
Source: Core Rulebook 3E, p. 66
A monstrous creature's close combat attacks are so powerful that they ignore armour saves entirely.