A credit invoice has a negative amount that the customer should get back. You can create credit invoices in the same ways that you make other invoice rows, in the field where you enter the price, you write a hyphen (-) in front of the amount, for example "-100".
When invoices are created you get the choice to include negative invoices or save them for later. That way you don't need to create and send an invoice where the customer should get money back, but instead the credit row will be used when there is a debit row to deduct the amount from.