A pinned piece is a piece that is subject to a pin, either on a rank (horizontal), file (vertical), or diagonal. A piece that is pinned against the King cannot legally move. A piece that is pinned to its Queen cannot move for the loss of the Queen. Pieces can also be pinned against a Rook or any other unprotected piece.
A pinned piece is quite weak and can sometimes be lost. The attacker will often try to “double up” on a pinned piece. Since it cannot move, you can attack it with another piece (or more than one). The defender will try to “unpin” a piece, such as by protecting the pinned piece, escaping from a pin, using a counter-pin, or some other methods.
Related Chess Tactics
Read more about these related chess strategies: