After the payment deadline, the outstanding balance on your credit card will accrue interest. The annual percentage rate (APR) that you’ll be charged is listed on your cardholder agreement as well as your closing statement.
Interest on outstanding balances will compound until the balance is paid off in full. So, if your balance increased last month due to an interest charge, you’ll now pay interest on top of the old balance plus the interest.
This can often mean your rate will increase to 30%+ depending on the card. So when your balance starts compounding (as stated above) at a high rate like this, your outstanding principal balance will grow very very fast.