A pin is a type of chess tactic in which your opponent cannot move a certain piece because doing so would expose a more important piece. As you will be see in many games, it can used within the first five or ten moves to gain a positional advantage so it is a nice tactic to be able to employ.
Examples of Pins
In the example to the left, the G5 bishop is pining the E7 knight because if the knight were to move then the the black queen would be under attack. This tactic immobilizes the black knight.
This tactic can be very beneficial in gaining material and beating your opponent. In this example the black king is in check by the white queen. Black HAS to move its king to a different square, it cannot move the rook to block the check because the rook is being pinned by the bishop. If the rook were to move, then the King would be in check by the bishop. Once the king moves away from the rook, white can easily capture the rook and move one step closer to checkmate.
Sherwood Chess 