This is the top-level category; looking at it another way, it is the root category.
Ideally, every other category should be a subcategory of at least one other, and every article should be in at least one category; many will fit well into two or more (e.g. location, year, and subject-matter).
In addition, no other category should be left without a category of itself, outside of this one. Categories that lack other categories can be found here.
A full list of existing categories can be found at Special:Categories.
To add a new category to a page, just add
[[Category:Categoryname]] to the end of the article or existing category page.