Help:Categories
Categories are pages that group together pages which share similar purposes, subjects or other qualities. To add a page to a category, add it as a link at the bottom of the page. Some templates may also assign categories when added to a page.
Hidden categories are just that; categories usually not displayed on a page. This is done only for maintenance categories, like Category:TODO. To mark a category as hidden, add the magic word on that category's page.
If you want to simply link to a category on a page, put a : at the start of the page name, like this: [[:Category:Shaders]].
It may take some time for a page to appear or disappear from a category. Try adding ?action=purge at the end of the URL to nudge the server to do it faster. The wait time may be even longer if assigned through a template.