This is the top category of the wiki; looking at it another way, it's the root category! Every other category and every other page should be in at least one "parent" category, and most will be suitable in two or more. Categories help people find pages when they don't know the exact page names.
To add a new category to a page, if you are in "Visual Editor" mode, you can click the Category button at the bottom of the page and type in the name you'd like your category to be called.
A full list of existing categories can be found at Special:Categories.