New to volleyball -- What's a libero?

[Scrapper1]

Originally Posted by FMadera

Libero may serve in only one rotation on the lineup

I don't know what this means (again, sorry, but I don't have a rulebook yet).

Libero may not attack a ball if at the moment of contact, the ball is entirely above the net.

Even from the back row? So the libero can't ever spike a ball that's above the height of the net?

If the libero finger sets while in the attack zone, the next contact cannot be an attack contacted from entirely above net height which is then completed. This attack would be illegal.

My only question here is "Why?"

There must be one rally between libero replacements, unless the libero is on the court and will replace into right back in order to legally be the next server.

So the libero can just go out of the game and come right back in to be the next server whenever he/she wants? You could just have the libero serve every time.

Obviously, I just don't get the whole concept of the libero. Can you explain in simple terms what the libero's purpose is? It doesn't make a lot of sense to me yet.