A second username name for the same poster. They are not permitted here. Originally, it referred to a sockpuppet, a second identity used by a poster to agree with himself in a thread and look like he was getting more support. Here it generally means someone who registers a new name to come back after being banned, or sometimes because they want to express unpopular opinions they don’t want to be identified with under their main name (or because they’ve embarrassed themselves and want to distance themselves from that name).