The yellow graphic first down line, that we all now see on every broadcasted football game, is not on the actual playing field.
It's only a graphic on TV, but we love and use it to know when a player makes, or is close to a first down.
That helpful tool we all love and use at home, can now be projected right onto the playing field, for everyone to see and use.
The players, coaches, referees and the fans in the stadium will then see what we all see at home on TV.
The goal line is visible, but the first down line is not. There are more first down calls in a football game, than goal line calls.