Categories is an overview all of your business categories and their balance. This is can be useful to see if your balances appear incorrect and need be analysed.
New categories can be added if required and unused categories can be deleted (or hidden if they have been previously used but no longer have a balance). There is also the option to see all categories, not just those 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.