Hives are a group of developing Volatiles which survivors must kill to win a multiplayer match against the Night Hunter.
The Hives are found throughout the multiplayer matches when the Night-Hunter invades the survivors game. Hives contain Undeveloped Volatiles which will soon develop unless the survivor kills them. Hives contain large amounts of shredded corpses and sometimes other Virals from biters to goons to even developed Volatiles. They are also seen in The Following.
The undeveloped Volatiles within the Hive do not attack the player but can take a large amount of hits depending on the players weapon. But its what's within the hive and what's coming to defend it that can and will attack you. The survivors will often need to kill the inhabitants of the Hive then attack the undeveloped Volatiles while also looking out for the Night hunter.
Another strategy is to throw down UV flares next to the hive to weaken it. That way, the hives take less time to destroy and their health won't regenerate.