The simple version:
If a player runs around all defenders, gets in front of the goalkeeper, and receives a pass from a teammate farther away from the goal, that player is offside.
To avoid being offside, don't run behind the "last defender."
This rule is important to understand as being Offside can lose your team a good scoring chance.
Everyone should remember the above ‘Simple version’
The long version (warning: harder to understand):
A player is in an offside position if she is nearer to her opponent’s goal line than both the ball and the second to last opponent unless:
- she is in her own half of the field of play, or
- she is level with the second to last opponent or two last opponents.
A player shall only be declared offside and called for being in an offside position, if, at the moment the ball touches, or is played by, one of her teammates, she is, in the opinion of the referee:
- interfering with play or with an opponent, or
- gaining an advantage by being in that position.
A player shall not be declared offside by the referee just for being in an offside position, or if she receives the ball direct from a goal kick, corner kick, or a throw-in.
If a player is called offside, the referee shall award an indirect free kick, which shall be taken by a player of the opposing team from the place where the offside occurred, unless the offense is committed by a player in the opponent’s goal area, in which case the free-kick shall be taken from any point within the goal area.