Chess PatternsPin
A piece can’t move because it would expose something more valuable behind it — often its own king. Pinned pieces can’t run, and often can’t defend either.
How it works (spoiler)
The bishop steps between the king and the queen. The queen can’t leave the diagonal without exposing the king, so the best it can do is take the bishop, and the pawn takes back. A pin against the king is absolute: the pinned piece may not move off the line at all.