In the example we just reviewed on skewers, you saw that it's sometimes to your advantage to break a pin. When a knight is pinned, but it can still legally move, we say that it's a "RELATIVE PIN."
In the example just above, Black has just played Bg4. This is a relative pin, and white has a special idea. White played 5.Nxe5 Bxd1 (if 5...dxe5 6.Qxg4) 6.Bxf7+ Ke7 7.Nd5 checkmate!
In the second example just below, the knight is pinned to the king. These are called ABSOLUTE PINS" because the knight absolutely may not move.
When your knight is pinned, you will almost always want to find a way to "UNPIN" your knight. Often, you will develop a bishop behind the pinned knight. Sometimes, you will "challenge" the bishop by attacking it immediately with a pawn. In the diagram just above, white will soon play a2-a3 to challenge the Black Bb4.