Categories can be used on Expenses & Subscriptions. They help separate the data so you can see how much your expenses are for going out to eat ETC...
The first step to create a category is to go to the Categories on the Dashboard:
This will be the same on mobile as well. You will need to click on the create category button:
You will fill the Name & About field in then click the Save button.
You have now created a category to be used in expense or subscription. You can now see the categories in the Categories home.
You can now click on your category to view it.