Credits are a form of discount. Credits are automatically used when you subscribe for a plan. We'll only charge your credit card when your credit balance runs out.
As an example, suppose your team is on the "Team Plan" with 3 users, that means your monthly subscription rate is $54 (3 x $18). You also have $10 in credits. So when we charge your card, we'll only take $44.
To see how much is your credit balance, go to Billing & Payments in your settings. You will see this stripe on top of the page: