Today it means: Pertaining to a rule concerning how to end a motorsports race, which requires that the final two laps of a race must be raced under green flag conditions, so that any red flag or (yellow) caution flag condition on these two laps will extend the race, to allow for a racing finish. Full definition →
From green + white + checkered. From the colors of the status flags, a green flag (clear racing condition), a white flag (last lap flag / bell lap flag), and a (black-and-white) checkered flag (finish flag), used in combination.
Etymology from Wiktionary, CC BY-SA 4.0.