Effy got it right. It's a preauthorization to guarantee the vendor that the card being used actually has a minimum amount of funds available to make the purchase. At practically any gas pump, it will preauthorize and put a hold on a certain amount of money so that if you get up to that amount from the pump, they know that the card you are using can actually pay for it. On some pumps, they will actually cut off when it reaches the preauthorized amount and you will have to re-swipe your card. If you don't use the preauhtorized amount, then it will be credited back to your card, or un-held. But sometimes that can take as long as 24-48 hours to happen. It's common practice for most places that take cards for sales that are variable at the point of service.