An outstanding authorisation is when the estimated value of a purchase (eg. Hotel stay) has been held up for a period of up to an estimated six days to allow the merchant to process the transaction.
It can also be used as a security measure for transactions such as Hotel stay or Car rental. In this case, after six days the amount drops off the system. Whilst the authorisation is in place your available credit balance is reduced by that amount, but not the balance owing on the card.