Mediawiki allows you to put articles into one or more categories. The wiki automatically keeps track of all categories on the Special:Categories page.
Categories are similar to the folders you might have made in a fileshare to group documents together so you can find them later. They are better than folders, though, because you can put articles into many categories so you don't have to decide if you want to e.g. file by person or by concept. For example, the article about a section head would be in the section head category, but also in their section category.
This is helpful to find and group articles. For example, you might be looking for the person in charge of dermatology but for some reason you can't find it because you don't know their title "section head". If you find another article related to dermatology you will see at the bottom that it is in the "dermatology" category. If you click on that category all other articles in that category will be listed, one of them being the one about the section head.
To add a category to an article add the following to the bottom of the article while editing:
[[Category:<replace with [[:Special:Categories|existing]] (if available) or new category>]]
If the category you use does not exist yet, the wiki will create it and group all further articles with the same category on that category's page. Check the Special:Categories page before you create a new category to make sure a similar one does not already exist.
See also: Links on the wiki