The Pin is a very important tool that every chess player should have in his or her repertoire. The word pin literally means that something cannot move. For example, if a poster is pinned to wall, it cannot move. In wrestling, a pin is when your opponent cant move. In chess, a pin is when a piece cannot move.
The Bishop on H8 is pinning the Knight on C3 with the King on A1. The Knight cannot move because it would leave the King in Check. (An Illegal Move)
Creating a pin in chess can be useful, but it does not always win a piece. Above, the King from A1 can move to B2 to protect the Knight. Whenever you know a chess piece cannot move, it is always a good idea to put pressure on it, and attack it with more chess pieces.
The Bishop from H6 can move to G7 to pin the Knight. The King is expected to protect it by moving to B2.
The Bishop from H6 can move to G7 to pin the Knight. The King is expected to protect it by moving to B2. The Black player can then win the knight by attacking it with the pawn on B5 by moving it to B4. The white player then does not have to defend the white knight, and cannot move it either because it is pinned. Remember: when a piece is pinned, take advantage of it by putting pressure on the pin.
The White Player has no way to save his or her Knight.