A pending transaction is a payment which has been authorised but is still waiting for the merchant's confirmation or reversal. The merchant has 7 days to either claim or cancel the payment. It can take up to 15 days for e-commerce pre-authorisation or deposit payments such as hotel reservations and car rental deposits.
Sometimes a declined transaction might appear as pending. In this case, the funds will be automatically returned to your available balance after 7 days.