Credits are a form of discount we sometimes offer our users. 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 "Garage Plan" with 3 users, that means your monthly subscription rate is $26.97 (3*$8.99). You also have $10 in credits. So when we charge your card, we'll only take $16.97
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: