Categories are managed at Organisation Settings / Finance Settings.
Categories are very handy items that are used primarily for Finances. They allow Organisations to allocate items to appear under those Categories.
There are a number of suggested Categories in the default setup of TidyHQ, but you may add as many as you require.
If you do not want a Category, you can "merge" it with another - the data attached to this category will be moved to the primary category. The secondary will then be deleted.