A payment can be left pending if it has been authorised, but has not been confirmed by the seller.
The seller has up to 8 days to confirm or cancel the payment. It might take a little longer in cases where there is pre-authorisation in e-commerce, or hotel reservations, for example.
Sometimes, a transaction that has been rejected can appear as pending. In this case the money will automatically be returned to your account after 8 days.