Board Thread:Support Requests - Getting Started/@comment-31783855-20170622082230/@comment-168424-20170622182445

Categories have 2 parts:
 * The tag (or wikitext code) you add to the pages to make them show up in the category list.
 * The category page itself that lists the pages in the category with an optional description that shows up at the top.

The help page linked above gives details on how to use this system.

People new to MediaWiki based wikis often assume you can add page to the category from the category list page itself or make a category page and add pages to it by clicking a button or something, but it doesn't work that way unfortunately.