The category is a broad label assigned to each agenda item. By default, the category is simply derived from the file name, but you can also specify it with a special line in the buffer, like this:
#+CATEGORY: Thesis
If you would like to have a special category for a single entry or a (sub)tree, give the entry a ‘CATEGORY’ property with the special category you want to apply as the value.
The display in the agenda buffer looks best if the category is not
longer than 10 characters. You can set up icons for category by
customizing the org-agenda-category-icon-alist variable.