The solid green flag is usually displayed by the starter to indicate the start of a race. During a race, it is displayed at the end of a caution period or a temporary delay to indicate that the race is restarting.

The waving of a green flag is almost universally supplemented with the illumination of green lights (resembling traffic lights) at various intervals around the course, particularly on ovals.

If the race is not under caution or delayed, it is said to be under green-flag conditions. However, the flag itself is typically not continuously waved by the starter. No flag displayed at the starter's stand implies safe, green-flag conditions. At all times, however, the green lights remain lit. Wikipedia