The main drawn of the update is Frontier Defense, a new co-op mode for up to four players. Your job is to defend a valuable objective while gunning down increasingly difficult waves of AI enemies.
The Titanfall update also adds two other game modes, Marked for Death Pro and Deadly Ground. In addition, it introduces sudden death for capture the flag to avoid ties, and more Titan insignias. You can see a full rundown of what's new in the update at Respawn's website here.