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
(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
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.