It's called the libero. In layman's terms, it's because they have different substitution rules than the rest of the players (come and go more freely, without counting against substitutions). They can only replace a back row player, and the player that they replaced can only come back in to the libero's position.
Libero can't start, but you can switch your libero every set.
They're basically a designated defensive player. The libero must start in the back row, cannot hit the ball at any point above the net, cannot block or attempt to block, and cannot perform an overhead set from in front of the attack line (that line a couple feet from the net).
It can get a lot more technical than that, but I think that's about all the casual volleyball fan needs to know about it. Someone correct me if I missed anything/got anything wrong.