A payment can be left pending if it has been authorised, but has not been confirmed by the seller.
The seller has up to 10 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 10 days.