Linesman or more properly “Assistant Referee” is a role in English football which works alongside the main referee to make sure football matches follow the rules. A linesman runs along both long edges of the football pitch, following the game play and may raise their flags when a foul is committed, when the ball goes out of play, or when a player is offside.
Ted wasn't happy with a linesman who officiated the Premier League game between AFC Richmond and Manchester City F.C. after he called offside when Sam passed the ball through to attacking players. Ted wondered how his player was offside and indeed wondered what the offside rule was, which was explained by Coach Beard later in the game.