Customer Credits
Customer credit comes in the form of a credit memo. You can apply the amount from a credit memo to an invoice to lower what the customer owes you.
For example, somebody returns two widgets to you that they originally bought for $100 each. When you receive the widgets, you issue them a credit memo in the amount of $200. Later, they purchase five widgets from you. The invoice total is $500, but when they apply the $200 credit memo, the total they are due to pay is only $300.