The car rental company will reserve a security deposit amount (as specified in your car rental agreement) against your credit card for damage that you might incur during your rental period.

 

At the end of your rental period when you return the car, if no damage has occurred, the rental company will reverse and release the security deposit amount from your credit card.

 

If I damage the car, will my credit card be charged? provides further information about the process should damage occur while the rental car is in your possession.