Categories are how you make sense of where your money is going — "Groceries," "Rent," "Salary," and so on. They're organized in two simple levels: broader groups (like "Food & Drink") that hold more specific subcategories underneath them (like "Groceries" and "Restaurants"). When you record a transaction, you'll always pick one of these specific subcategories — the groups themselves exist just to keep things tidy and let your reports roll up nicely.
Why categories matter
Categories are how you make sense of where your money is going — "Groceries," "Rent," "Salary," and so on.