Trample is a static ability that allows creatures to deal extra damage to the defending player, even after being blocked.
When a creature with trample attacks and is blocked by an opponent's creature, if the blocking creature has less toughness than the attacking creature has power, any additional damage not dealt to the blocking creature can goes through still damage the defending player or planeswalker.
Andy attacks with his Garruk's Horde and Nikki blocks it with her Canyon Minotaur, normally this would mean Nikki would take no damage, but as Garruk's Horde has trample, she can have it deal 3 damage to the Canyon Minotaur and deal the remaining 4 damage to Nikki.
- 702.19a. Trample is a static ability that modifies the rules for assigning an attacking creature's combat damage. The ability has no effect when a creature with trample is blocking or is dealing noncombat damage.
- 702.19b. The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that's being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that's actually dealt. The attacking creature's controller need not assign lethal damage to all those blocking creatures but in that case can't assign any damage to the player or planeswalker it's attacking.
- 702.19c. If an attacking creature with trample is blocked, but there are no creatures blocking it when damage is assigned, all its damage is assigned to the player or planeswalker it's attacking.
- 702.19d. If a creature with trample is attacking a planeswalker, none of its combat damage can be assigned to the defending player, even if that planeswalker has been removed from combat or the damage the attacking creature could assign is greater than the planeswalker's loyalty.
- 702.19e. Multiple instances of trample on the same creature are redundant.