Welcome to Tier 0.
This is our top-level category; looking at it another way, it is the root category with all other categories going below it. Tier 0 is the set that contains everything, even itself.
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).
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 functioning categories can be found at Special:Categories, a full list of categories with nothing within them can be found at Special:UnusedCategories. For uncategorized categories look in Special:UncategorizedCategories.