A pending card payment is a card payment that has been authorized, but not yet cleared. Payments are usually authorized as soon as you make them, and you will see your available balance deducted.
Most payments clear within 1-3 business days, but it can take longer. Payments made as ‘pre-authorisations’, such as card swipes when checking into a hotel or using a car rental service, are designed to give the merchant more time to charge you, so they can sometimes take up to 31 days to clear.