Some creatures are so massive that their sheer bulk is a threat all of its own.
Source: Warhammer Fantasy: 8th Edition
A model with this special rule can make a Stomp in addition to its other close combat attacks. A Stomp has thespecial rule, and inflicts 1 automatic hit, at the model's Strength, on one enemy infantry, war beasts or swarm unit in base contact with the model. This represents the creature crushing the foe beneath its ponderous feet, or knocking their broken bodies aside with one sweep of its mighty tail (probably while it's roaring in a most intimidating fashion).