The Beer Card is the ♦7. It is not part of the official rules of Bridge, but there is a tradition among some (typically younger) players that if the declarer succeeds in making the contract and wins the last trick with the ♦7, dummy must buy the declarer a beer of the declarer's choice. In the same way, if the opponents defeat the contract and one of them wins the last trick with the ♦7, the opponent who wins the last trick is bought a beer by the other opponent.
A fuller explanation is provided by the Bridge Guys.
