A linesman, also known as an assistant referee, is an official in sports like soccer and American football who helps the main referee make decisions during a game. Their primary responsibilities include signaling when the ball is out of play, indicating offside positions, and monitoring player conduct along the sidelines. Linesmen use flags to communicate their decisions to the referee and players. They are positioned along the touchlines or sidelines, providing an additional set of eyes to ensure fair play and adherence to the rules. Their role is crucial for maintaining the flow and integrity of the game.