The ledger account codes, or chart of accounts, helps you and your accountant to track and break down your sales. Go to Settings -> Payments & Invoicing -> Chart of accounts.
Your accountant might prefer certain account codes. But the basic principle is to categorize the different products you sell and the other accounts involved (payments, taxes, etc).
You may use the same account code for all your products, for example “4000 Sales”. But if you use unique codes for certain products, you will be able to track different types of income separately.