The five offensive linemen who line up on the line of scrimmage are not allowed to be down field on passing plays. This means that the O-line is not allowed to cross the line of scrimmage on passing plays; the penalty for doing so is a five-yard penalty.
While offensive linemen are not allowed to cross the line of scrimmage on passing plays, the rules are different for screen plays. On screen plays the O-line is allowed to cross the line of scrimmage to go out and block because the ball is still being thrown behind the line of scrimmage even though it is a forward pass. This is a common penalty for teams with running quarterbacks because the O-line might think the quarterback is about to take off and run when he is actually rolling out to throw.