Pinning

A pin is where a piece cannot move to other squares because the king is behind it for example:

In this position, the queen cannot move to other squares because the king is behind it and the bishop is threatening the queen. A pin can be very dangerous at some points because it usually happens to a queen more often. Here is one game I will show you where the queen was pinned and black lost a queen:

At the 6th move, black lost its queen because of the pin, and black had no other way to defend the queen. The example will a puzzle (LET'S SEE IF YOU UNDERSTOOD THE PIN VERY WELL).