A bishop or rook pins an enemy piece to a more valuable one behind it. The pinned piece can't move without losing the bigger one. You miss the pin or fail to pile on.
vs casual game at 1050: black knight pinned to the king, white had a pawn push that wins it. Played a different move and lost the moment.
vs puzzle: pin a defender to the queen, then take the undefended piece. The classic two-move tactic.