A sock-puppet is a derogatory phrase for a secondary profile created so that the person who created it can say things online that, for whatever reason, he doesn't want to be associated with in his primary profile.
Most often people use them to troll in forums, or to agree with something their primary profile posted so as to sound more convincing.
But sometimes people create "sock-puppet" profiles on HubPages because their primary profile has been used to post mainly humor, for example. They want to try writing something a little more serious, but they don't want their writing to be dismissed based on their earlier humorous pieces. So they create a new profile for their serious writing.