Similar to a zerg rush, a zerg wave is an attack which is strong because of the sheer number of people involved in the attack. The term "zerg wave" is commonly used in FPS games, but it has been used in other contexts as well. The term "Zerg Wave" can be used as both a noun and a verb.
We totally got zerg waved by those noobs." | "Look out, here comes a Zerg Wave!" | "They're zerg waving us over here!