A pin occurs when a piece is attacked and if it moves a piece of greater value will be under attack.
In the diagram below, the White Bishop is pinning the Black Knight. If the Knight moves the Queen will be captured. At the same time, the Black Bishop is pinning the White Knight. However, this time the Knight cannot move because doing so would put the King in check.
In the diagram below, the Black Knight is pinned against the King. The Knight is unable to move because if it does it would expose the King to check. If a piece is pinned, attack it. White moves her pawn to d3 and the Knight is lost!