If somebody has to ask, usually several don't know, so:
Sockpuppet, or just sock, is slang for double registrations. People may register several identities on a forum.
There can be some legitimate reasons for that. You may want to have different personalities, like one for very serious debates, and another for having fun.
A more usual reason for doing it, and the reason it has become illegal here, is to circumvent suspension or banning. A troll-type user may register several different identities, then leave most as sleepers. Then when they get suspended or banned, they can switch to another ID and keep posting. They usually soon give themselves away, though.
ETA: Another use we have seen of socks is to get apparant support for your viewpoints by having one or several sockpuppets that pipe in and agree with you in discussions.
Hans