What is offside in football?
An attacker is offside if, at the moment a team-mate plays the ball, they are nearer the opponents' goal line than both the ball and the second-last defender (usually the last outfield defender plus the goalkeeper) — and then become involved in the play.
Being in an offside position is not an offence by itself; it is only penalised if the player then plays the ball, interferes with an opponent, or gains an advantage. A player cannot be offside in their own half, from a goal kick, throw-in or corner. The punishment is an indirect free-kick to the defending team.