Pin
A pin is a line attack that freezes a piece because moving it would expose a more valuable piece behind it.
If the piece at the back is the king, the pin is absolute: the front piece is illegal to move. If the piece at the back is a queen or rook, the pin is relative — you can move the front piece, but you will usually drop material.
In the diagram, Re1 lines the rook up with the knight and the king. The knight cannot step away without leaving the king in check, so White can pile on e7 or simply keep the piece out of the game.
When the coach flags a pin during a game, look at the whole line, not just the two pieces that are touching. The winning idea is often to attack the pinned piece a second time.