Pin
A piece can't move because something more valuable is behind it.
Example
The knight sits in front of its own king.
- 1. Bb5: The bishop pins the knight to the king: it can’t move.
- 1... h6: Black can’t do anything about it.
- 2. d5: Now attack the pinned piece with a pawn.
- 2... h5: It still can’t move…
- 3. dxc6: …so it’s won.
Practice the pin free on ChessScout.net