That player is called the "libero". They are a defensive player, and there are some limitations on what they are allowed to do:
-they are not allowed to serve
-they may no attack the ball (spiking the ball if it's above the plane of the net)
-they may not come within 3m of the net (there's a line there called the "attack line") and use their hands to set an attacker.
They are allowed to come in for any player though. Volleyball substitutes can normally go in/out for one player.
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