From Scratchpad

Categories are used to group pages of the same wiki. If you don't use a category tag, your pages might be orphaned and not linked to your project.

The home page of your mini wiki will tell you which category tag to add. For example, if your mini wiki was about dogs, you would type [[Category:Dogs]] on all of the pages that were part of that wiki.

See Wikia:Help:Category for more help.