A pin happens when a piece is lined up in front of something more valuable, usually the king, along a rank, file, or diagonal. The pinned piece can't move without exposing the more valuable piece behind it, so it becomes effectively frozen.
Pins are everywhere in the opening. A common one: a bishop develops to attack a knight that's directly in front of the enemy king, and the knight can no longer move without walking into a check.
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h
Black just played Nc6. The knight sits directly in front of the king on e8.
Which move pins the knight on c6 to the king?
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h