Pin is tactical motif. Pin is a situation brought on by an attacking piece in which a defending piece cannot move without exposing a more valuable defending piece on its other side to capture by the attacking piece. Piece that can't move, because pin is ''pinned'' There are 2 kinds of pin: relative and absolute pin. Absolute pin means that the piece can not move because the king is behind the piece which is pinned. Relative pin means that the piece can't move, because other piece of higher value is behind it. In relative pin, player can move pinned piece, but with the risk of losing another piece.
Black knight is pinned and can't move, because bishop will capture king. This is position from Ruy Lopez: Steinitz Defense(1. e4 e5 2. Nf3 Nc6 3. Bb5 d6)
White to move
It's relative pin, because white will lose queen if move their knight.