In chess, a pin is a move which forces one of the opponent's pieces to stay put because moving it would expose a more valuable piece behind it or a situation where such a thing has happened.
The black rook is pinning the white knight to the queen. This is a relative pin; white is unlikely to move the knight because it risks losing the queen, which is much more valuable--but he still has the choice.
The act of breaking a pin is unpinning. This can be executed in a number of ways: the piece effecting the pin can be captured; a piece can be interposed between the pinning unit and the pinned unit; a piece can be interposed between the pinned unit and the unit to which it is pinned; or the unit to which a piece is pinned can be moved.
A pin that often occurs in openings is the move Bb5 (see algebraic chess notation) which, if black has moved his d-pawn, pins the knight on c6, because moving the knight would expose the king on e8 to check. A common way to win the queen is to pin her to the king with a rook, for instance with a white rook on e1, the black queen on e7 and the black king on e8.
See also: Chess terminology