Sockpuppets are fake accounts designed to review the author positively or their competitors negatively. Many authors use them to gain an unfair advantage and create a false positive profile for their book. There are several authors that are part of the community that have used them, though the most famous one that bragged about it, and brought it fully to the fore is the author mentioned in this Guardian article. It should be noted that since this article came out in 2012, while there are other authors that do this, the author in question (who we are not naming so they get no attention from this) has since came back and said that he regrets it. But most authors don’t get caught, which is why Amazon cracks down so heavily on what they consider to be possibly fake reviews.
While this first came to the fore in 2012, it still continues to this day, by some authors. It and reading farms are a cottage industry of cheating readers.
Sockpuppets are not only highly suspect and make indie authors look bad, but they are part of the reason that Amazon is cracking down and removing reviews.