Tactical motifs
Pin
A piece cannot move because a more valuable one sits behind it.
What is the pin?
A pin freezes an enemy piece: it cannot legally or sensibly move because doing so would expose a more valuable piece (or the king) directly behind it on the same line. Only bishops, rooks and queens can pin, since they attack along lines.
Why the pin matters
A pinned piece is a piece that has stopped working — it cannot defend, capture or run. That is why "pile up on the pinned piece" is one of the oldest pieces of chess advice. Absolute pins (against the king) are total; relative pins are powerful but breakable.
How to use it to win
When you want to win a defender, ask whether you can pin it first so it cannot flee, then hit it again with a pawn or lesser piece. When you are pinned, look to challenge or block the line, or to create a bigger threat that makes the pin irrelevant.
Try an example puzzle
A real pin puzzle from the app. White to move — find the idea, then reveal the line.
6 
4 
c Show solution
1. Ng6+ Kg8 2. Qh8#
Drill the pin on thousands of real puzzles, matched to your rating.