When you use your debit card in situations where the final amount is unknown, such as at hotels, gas stations or restaurants, the business can ask your bank to place a hold on your checking account for an amount larger than your purchase. This is done to ensure that you have enough to cover the final amount.
No money leaves your account until the purchase clears but the hold is active until the actual transaction is settled, which most often is within 24 hours, but can take up to 3 days.