A pin is a situation where a piece cannot be moved because it would either put the player's own king in check, or because it would expose a valuable piece to attack from a less valuable piece. In the first example, Black's knights are pinned, one to his king and one to his queen, by white's bishops.
Pinning a piece is a great way to immobilize it, and often it can be as good as temporarily removing the piece from the board. In this next example, white's pawn is pinned to the king, so black can immediately take the other pawn (on the left) with his bishop, and white cannot take the bishop.
In this final example, notice that what appears to be a pin actually isn't, because white can "buy a move" by putting black in check with the knight. Black must respond to the check on his turn, allowing white time to move the rook out of the bishop's line of fire.
