It's the top level of the category, in other words, it's the starting point for most articles until they are further categorized.
A full list of existing categories can be found at Special:Categories.
To add a new category to a page, just add
[[Category:TheCategoryName]] (where TheCategoryName is the name of the category) to the end of the page. You may also add categories to existing category pages to link relationship among one another, in which it helps to maintain a good category tree.
This category has the following 6 subcategories, out of 6 total.