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 notice, pinning isn’t technical a tactic in itself because it’s not a sequence of moves. It can be leveraged in a tactic, consider the following example:
is a typical situation in which a pin leads to a tactic.
d4 will win material because the black knight is the victim
of an absolute pin.