Sometimes customers try to pay for an order through our web site and their credit/debit card payment fails. This is usually because they failed our payment gateway security checks, such as ensuring the address given matches the registered card address. When this happens banks often place what they call an "Authorisation Hold" on the account for the transaction amount, effectively freezing those funds for a short period of time. We (the retailer) have not received this money and have no control over this process. These funds are usually returned to the account after 24 to 48 hours, but can take as long as a week. 

We are often asked why a transaction is showing on a card when the payment failed, and Authorisation Holds are the reason. As a retailer, we have no control over the amount or duration of these Authorisation Holds. We do not request or place these Authorisation Holds.We cannot release an Authorisation Hold early. This is all totally outside our control and is managed solely by the payment/bank card providers.

This explains authorization holds better than I can: https://en.wikipedia.org/wiki/Authorization_hold