See Also Payback
Trample is a quality shared by all Large and XL creatures; it is a built-in game mechanic, therefore it is not explicitly stated on cards. Large units trample Small creatures, because they are two sizes bigger. XL units, being two sizes bigger than medium creatures, trample M sized creatures as well. And of course, as they are even bigger than small creatures, XL creatures trample them. A default damage of 10 is done to each trampled individual unit, this number is multiplied by the number of individual creatures in a x6 or x4 squad.
Payback cards do damage to units that trample them. However, some Payback units have the Steadfast ability, which prevents them from being knocked back, so 'attempt to trample', to be specific. The Payback damage is still inflicted, even though the units are not knocked back.