A sockpuppet is a wiki account created for the purpose of convincing wiki readers that the account is owned by someone other than the person (or really, identity) that created it. For example, if Alice posts on the wiki using the username "Alice", she may wish to use a different username to post comments that do not appear to come from her. To achieve this, she may create another account called "Bob", so that wiki readers believe that Bob posted comments rather than Alice.
There are various reasons why someone might create sockpuppet accounts. One is to make it look like a lot of different people think a particular business is really great, or to make it look like a lot of different people think a particular business is really awful, when in fact it is just one person's opinion. Or sometimes people will argue with themselves just to mess with the rest of us.
Creating sockpuppet accounts is discouraged and is often easily detected through various means. When a sockpuppet is detected, alert wiki readers will often note that the account is most likely from another user, identifying that user, and the reason why they believe the poster is a sockpuppet. Because of this, postings by sockpuppets generally have very low credibility, and users who create sockpuppets may have their own credibility reduced in the view of other wiki readers. Comments from sockpuppets may even be deleted if circumstances warrant.
The wiki even has a nifty Include which can be copied and pasted onto user pages if sockpuppetry is suspected: