Trample

Trample is static ability that lets a creature deal excess combat damage to a defending player or planeswalker while attacking.

Comprehensive Rules
702.19. Trample

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. (See rule 510, “Combat Damage Step.”)

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.
 * Example: A 2/2 creature that can block an additional creature blocks two attackers: a 1/1 with no abilities and a 3/3 with trample. The active player could assign 1 damage from the first attacker and 1 damage from the second to the blocking creature, and 2 damage to the defending player from the creature with trample.
 * Example: A 6/6 green creature with trample is blocked by a 2/2 creature with protection from green. The attacking creature’s controller must assign at least 2 damage to the blocker, even though that damage will be prevented by the blocker’s protection ability. The attacking creature’s controller can divide the rest of the damage as he or she chooses between the blocking creature and the defending player.

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.