A pin is one of the basic tactics in the game of chess, which, when effectively employed will put the opponent on tenterhooks and you gained to capture one additional piece in most cases.
As you may be aware, a tactic is a sequence of moves that are oriented towards a specific result in the game of chess, wherein the sequence results in either capture of a piece or capturing the King itself – that is checkmate.
A pin is an attack on a piece, which shields one of its own pieces from attack. The attack exists on a straight line of squares, be it a horizontal rank or a vertical file or a diagonal. Here, the piece, which is attacking the shielding piece, is called the pinning piece and the piece, which is shielding the other piece, is called the pinned piece.
In other words, when a piece is attacked and cannot move without exposing a second, usually more valuable, piece to attack, then it is said to be pinned.
Let me make it clear with an example. Your opponent’s king is at “e8” and the knight is at “c6” on the white diagonal “a4” to “e8”. Now assume that you are moving your white bishop to say “b5”. By this move, you are not only attacking the Knight, but also pinning it. Because, if the opponent’s Knight attempts to move from that square, the King in “e8” will be exposed and as such the Knight cannot move before safeguarding the King with other pieces or moving it out of the attacking diagonal. Your bishop is the pinning piece and the opponent’s Knight is the “pinned piece”.
There are two types of pins – one is absolute pin, and the other one is a relative pin. Absolute pin occurs when the piece shielded by the pinned piece is a King. The above given example is a case of “absolute pin”. When your pinning piece employs the absolute pin, the pinned piece becomes immobile.
In the case of relative pin, the piece shielded by the pinned piece is not the King but another valuable player in the game – something like a Queen or a Rook
Let me give an example for a relative pin. Your opponent’s Queen is at “d8” and the knight is at “f6” on the black diagonal “h4” to “d8”. Now assume that you are moving your black bishop to say “g5”. By this move, you are not only attacking the Knight, but also pinning it. If the opponent’s Knight tries to move from that square, the Queen in “d8” will be exposed. This is an example of “relative pin” where your bishop is the pinning piece and the opponent’s Knight is the “pinned piece”. The pinned piece in a relative pin can technically move, as against immobility in an absolute pin, though whether it is advisable to do so is something different.
As such, a pin is a very effective tactic in the armory of a chess player to gain advantage over the opponent’s pieces, as the probability of capturing one extra piece is always there.