Graveyard zerging is the act of overwhelming an enemy (usually an outdoor boss) by having players immediately run back to their corpse and resurrect during a battle, rather than use any sort of strategy or skill in the battle. The point is to keep a neverending stream of players attacking a target until it's dead.
This was once a major problem with the World Bosses, but Blizzard's solution was to give players a 15-minute debuff when they die which freezes/sleeps the player if they get too close to the World Boss. The Four Dragons and have this feature. becomes nearly immortal after 3 minutes of battle, and is healed each time he kills a player.
See also: Zerg