This is a weapon that requires two enemy pieces on the same line with a friendly long-range piece. Instead of two good guys and one bad guy on the line, as in a discovered attack, we have one good guy holding two bad guys hostage. Well, only one of them is actually held hostage, but they both have to be there.
The pin is more akin to a wrestling pin than to a sewing pin. In it, one friendly long-range piece looks at a powerful enemy piece with a less powerful enemy piece shielding it.
The Pinned Piece
This is the lesser enemy piece that acts as a shield to the more powerful or more important enemy. Some really good examples of pinned pieces come up in the Morphy versus Duke and Count game from 1857 that you saw back in Chapter 6 and again in Chapter 10. Here it is again with the pins pointed out:
1. e4 e5 2. Nf3 d6 3. d4 Bg4.
The White knight is pinned to the queen.
4. dxe5 Bxf3 5. Qxf3.
Now there is no more pin.
5…. dxe5 6. Bc4 Nf6 7. Qb3 Qe7 8. Nc3 c6 9. Bg5.
The Black knight is pinned to the queen. This pin stays around for a while.
9…. b5 10. Nxb5 cxb5 11. Bxb5+ Nbd7.
The Black knight is pinned to the king.
12. 0-0-0 Rd8 13. Rxd7 Rxd7.
Now it is the rook that is pinned to the king.
14. Rd1 Qe6 15. Bxd7+ Nxd7 16. Qb8+ Nxb8 17. Rd8 checkmate.
On move eleven of the game you just looked at, Black moves a knight into the way of the checking bishop and the king. This blocks the check, so is good. But it also puts Black into an absolute pin. That is, a pin in which the pinned piece (the knight on d7) not only shouldn't move, but can't move.
One of the best ways to take advantage of a pinned piece is to hit it again and again. If you want to break down your opponent's defenses, harsh measures are called for. So when you spy an immobile piece, such as a pinned piece, attack it again. This is much more effective than simply exchanging the pinning piece for the pinned piece.
Whenever a piece lies between your king and an enemy long-range piece, it is in an absolute pin. It cannot move out of the line of fire, since you are not allowed to place your king in check.