Often when you check into a hotel or hire a car the retailer will ‘reserve’ a set amount of funds from your account in case of any unexpected bills during your stay or hire.
If there are no unexpected bills the amount will be dropped from your account. This could happen after your stay or hire is complete, but could also happen earlier if the retailer decides.
Some online grocery retailers will reserve an amount until they have checked they have everything in stock and then adjust the final payment.