Categories are used to categorise transactions based on self configurable rules. These categories will be shown on your payments history on the Dashboard and will also be used to create beautiful statistics on the statistics page.
Here are also some default categories included with bunqDesktop installation from version 0.9.0 onwards. But you can of course delete them, recreate them or add your own categories.
To do this you will need to go to the categories section of bunqDesktop where you can edit, delete and add them: