When making a purchase with your checking account card, you'll swipe the card to authorize the transaction. This sends a request from the merchant to your bank confirming that your account is active and has the necessary funds available. If both of those conditions are met and the purchase goes through, a hold is placed on your account setting aside that amount. That pending charge remains until the merchant sends confirmation of the final transaction or until it falls off the account. The latter might occur after a few days if a transaction is canceled after the card is swiped.
Amount on Hold
The pending charge may be greater or less than the amount you actually spent, and that ultimately will be processed. A restaurant, for example, may only preauthorize the amount of the meal itself and not the tip. Your pending charge will therefore be less than the amount you finally are charged. On the other hand, a gas station doesn't know how much fuel your car needs when you arrive at the tank, so it might preauthorize a set amount like $50 or $75. If your gas bill is less, you'll have more money set aside in pending charges than you actually spent.