Every transaction starts with a pre-authorisation. This checks that you have funds available on your card to pay the merchant for their services. The actual transaction will come through within a few days. Until that time, the pre-authorised amount will be held on your card.
For example: if you open a tab at the bar, a fixed amount is pre-authorised to cover what you could spend. If your final bill is less than the pre-authorisation amount then it may take a while for the difference to be released back onto your card.