Signals the start or re-start of a race.  When displayed it indicates that the course is clear.

  Another car is following you very closely or about to pass you.

  An ambulance, service vehicle, or slow moving car is on the track.

 A slippery condition exists or debris is on the track.  

  Danger, slow down.  No passing from the flag until past the emergency area.

  Danger.  The session has been stopped.  Come to a stop at the side of the track.  

  There is something mechanically wrong with your car.  Proceed to the pit.

  Signals the end of the race or practice/qualifying session.