Categories is an overview all of your business categories and their balance. This is can be useful to see if balances look incorrect and should be addressed.
New categories can be added if required and unused categories can be deleted (or hidden if previously used but no balance). You can also show all categories, not just categories with balances.
From Categories you are able to enter adjustments. As a user, it would be rare that you use this function and so it is recommended to seek the advice of your accountant if you are unsure about entering an adjustment.