Credit cards are like loans. Every time you charge something on your card, you are borrowing money which you can pay back later over a period of time or in full when the balance is due. If you choose to pay money back over time, interest will be added to your account. This interest is also known as the APR and you pay that in addition to the purchase amount if you carry the balance beyond the first month.
The APR is an annual percentage rate which is calculated on a yearly basis. The higher the APR, the more you will pay. You can avoid paying this interest if you pay your balance in full each month. If you pay less than the full balance and carry the balance from month to month, the interest will accrue.