Tactics & checkmates
Pin
A pin restricts a piece because moving it would expose a more important target behind it. If that target is its king, the pin is absolute: a move that exposes the king to check is illegal. In a relative pin, moving the piece is legal but may lose material.
A practical example
A white bishop on b5 faces a black knight on c6 and king on e8. With d7 empty, moving the knight would open the diagonal to the king. The knight is absolutely pinned.
A common mistake
Treating every pinned piece as powerless. It still attacks squares, and a king move, exchange or interposition can break the pin.
Try this in your next review
Identify the target behind the pinned piece. Then look for another attack on that piece and check how the opponent could break the pin.
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