A quick look at the different types of pin that can occur during a game

In Chess, a pin is an indirect form of attack. The queen, the bishops, and the rooks can perform such an attack. Here is a pretty common example of how pinning looks like:

In this position the black knight on f6 is pinned by the white bishop on g5 (this is a common variation of the giuoco pianissimo opening). The fact that the knight is pinned means it can’t really move because that would lead to material loss. In this case, the knight shields the queen so this is called a relative pin. Here is another type of pin:

In this case, the black bishop in g4 pins the white knight on c3 which shields the white king. Moving the knight is not a legal move and this kind of pinning is called an absolute pin.

As you may have noticed, technically a pin is not a tactic in itself because it is not a sequence of moves. But it can be leveraged in tactics, consider the following example:

This is a typical situation in which a pin leads to a tactic. d4 will win material because the white knight is the victim of an absolute pin.