Pins

A pin happens when a sliding piece (bishop, rook, or queen) attacks a piece that has something more valuable standing directly behind it on the same line -- if the front piece moves out of the way, the attacker captures whatever's behind it instead. The pinned piece is stuck: it can still technically exist on the board, but moving it would be a mistake (or, if the piece behind it is the king, moving it would be flat-out illegal).

That second case -- when the piece behind the pin is the king -- is called an absolute pin, and it's the strongest version: the pinned piece cannot move at all, by the rules of chess, no matter how much the player might want to. A pin against anything other than the king (a queen, a rook) is a relative pin: the piece is legally allowed to move, but doing so hands over the more valuable piece behind it, so it usually shouldn't.

Pins are a favorite setup move precisely because a pinned piece is a piece that has temporarily stopped doing its job -- it can't defend other squares, can't capture, can't participate in the game, all while still occupying space on the board. Strong players routinely pile more attackers onto a pinned piece, since it has no way to escape or fight back.