Sentence:  To the unintelligent or careless, the great stumbling-block is Law 6, known as the <WSD>off-side rule</WSD>. It  is of itself a reflection on the bulk of young players that this particular provision is so little understood, or rather so misunderstood. The rule provides that when a player kicks the ball, or throws it in from touch, any one of the same side who at such moment of playing or throwing is nearer to the opponents' goal-line, is out of play, and may neither touch the ball himself, nor in any way whatever prevent other players from doing so, until the ball as been played, unless there are at such moment of kicking or throwing, at least three of his opponents nearer their own goal-line. 
Meaning of the word marked with <WSD> tags:  The prohibition of teammates closer to the opponents' goal than the person kicking or throwing the ball unless there are three opponents at least as close to their goal line.