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.
The "Content" category is intended for all articles about the subject matter of the wiki, whilst "Organisation" is intended for all administrative matters. 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.