Pin

A pin is when one of your opponent's pieces can't move without exposing a more valuable piece sitting right behind it — often the king or queen. The pinned piece is stuck acting as a shield. Pile more attackers onto it and you can often win it.

The bishop on g5 pins the black knight on f6 to the queen on d8. If the knight ever moves, Bxd8 wins the queen, so the knight is frozen in place.

Source for further study: Bain, Chess Tactics for Students, ch. 2.