Pins and Skewers: Two Sides of a Coin
The same trick, facing two ways
Pins and skewers both work by lining up two enemy pieces on a single straight line and attacking through them with a bishop, rook, or queen. The difference is only which piece stands in front. In a pin, the less valuable piece is in front, and it dare not move because something precious hides behind it. In a skewer, the valuable piece is in front, so when it flees, you grab whatever was sheltering behind it.
Because only the long-range pieces, bishops, rooks, and queens, can attack along a line, only they can pin or skewer. A knight or pawn can never do it. That alone is a good reason to value your bishops and rooks on open lines, where these threats live.
Absolute versus relative pins
A pin against the king is called absolute, because it is literally illegal to move the pinned piece, doing so would expose your own king to check, which the rules forbid. The pinned piece is nailed to the spot completely. A relative pin is softer: the piece in front shields a queen or rook rather than the king, so it can legally move, but doing so loses material. The opponent may sometimes accept that loss for a bigger gain, which is why relative pins reward careful calculation.
Pins are powerful because a pinned piece is a paralyzed piece. It cannot capture, cannot defend, cannot do its job. A favorite plan of strong players is to pin a defender and then pile more attackers onto whatever it was guarding, since the frozen piece can no longer help.
Using them and avoiding them
To exploit a pin, attack the pinned piece again with a pawn or another piece, it cannot run, so you simply win it. To escape a pin, you can block the line with another piece, challenge the pinning piece by attacking it, or unpin by moving the valuable piece behind to safety. And whenever you develop, take a half-second to notice if you are walking a knight or bishop into a pin in front of your own king or queen.