Pins are a very common tactic that restrict the movement of the opponent's pieces. A pin is created by attacking a piece in such a way that if it moves, it will allow the capture of a more valuable piece located behind it. Only long range pieces -- queens, rooks and bishops -- can create pins.
If the piece behind the pinned piece is a king, the pinned piece is not allowed to move, as it is illegal to put your king in check. This is known as an absolute pin. If moving the pinned piece would only allow a more valuable piece, such as a queen, to be captured, it is known as a relative pin.
In the diagram above, we have an example of an absolute pin. Although it is Black's turn to move, there is no way to save the rook on d5; attempting to move it would put the Black king in check because of the White bishop on b3. After Black moves his king, White can play Bxd5, winning the rook.