Lethal damage is damage equal or greater to a creatures current toughness. This is important when assigning combat damage, and especially damage from an attacking creature that is blocked by multiple creatures or an attacking creature with trample.

When assigning combat damage a player must assign what would be lethal damage to each blocking creature in order before assigning damage to the next blocking creature. Additionally, when a attacking creature with Trample is blocked the controller of this creature must assign what would be lethal damage to each blocking creatures before assigning damage to the defending player.

Here are two examples. An attacking Fangren Marauder is blocked by a Stone Golem and a Goblin Guide. When blockers are declared the controller of the attacking Marauder has to choose an order to assign damage to these two blocking creatures. When assigning combat damage, he would then have to choose to assign lethal damage to one before assigning any to the other. Assuming he chose the Stone Golem First, he’d have to assign four damage to it before assigning any damage to the Goblin Guide.

Similarly, lets assume that an attacking Duskdale Wurm has been blocked by a Canyon Minotaur. After blockers are declared the Canyon Minotaur is targeted by a Giant Growth. The controller of the Wurm would have to assign at least six point so damage to the blocking Minotaur and can assign up to one point of damage to the defending player.

One final aspect about lethal damage to point out is that any combat damage from a source with deathtouch is considered to be lethal damage. (The official rules state it must be a nonzero amount, meaning that it must be one or more points of damage.)