Learn
The pin
Freeze a piece in place.
A pin is the opposite of a skewer: you attack a piece along a line, but behind it sits something MORE valuable (often the king). Moving the pinned piece would expose the bigger one, so it's stuck — frozen in place.
When the piece behind is the king, moving the pinned piece is actually illegal — that's an 'absolute pin'. A pinned piece is a sitting duck: you pile a second attacker on it (a pawn is perfect) and win it, because it can't run. Bishops, rooks, and queens pin along their lines.
Why can't the pinned knight move out of the way?
Answer the question to keep going!