At the start of the NASCAR race, keep your eye on the flagman (a NASCAR official), who is perched above the race track at the start/finish line in a crow's nest of sorts. He's waving different colored flags at the drivers as they zoom by in their race cars. These are the various flags used and what they signal:
Green flag: The flagman waves this flag to start or restart a race. Green means go, so when a driver sees this flag, he slams on the gas pedal and takes off.
Yellow flag: A yellow flag means NASCAR officials have called a caution period because an accident or debris on the track makes driving conditions dangerous. When drivers see a yellow flag, they know they must slow down to line up behind the pace car and drive cautiously around the track until the track has been cleared.
Red flag: Drivers must stop on the track — in a designated area — when they see the flagman wave a red flag. It means it isn't safe for drivers to circle the track because of inclement weather or poor track conditions.
Black flag: When the flagman waves a black flag at a driver, that driver must get off the track and go to the pits immediately. He did something wrong or his car isn't fit to be on the track.
Blue flag with diagonal yellow stripe: This flag alerts a driver that a faster, lead-lap car is about to pass him and he must yield to that car.
White flag: This flag means that the race leader has one lap to go in the race.
Checkered flag: When the checkered flag waves, a driver has crossed the finish line and won the race.
Green–white–checkered flag sequence: If there is a caution during the final laps, this flag sequence announces that there will be a green-flag restart of a couple laps. A green flag signals the first lap of the restart, and the white flag signals the final lap that leads to the checkered flag. It was added in 2004 to help ensure a race doesn't end under caution. Races get only one chance for a green–white–checkered finish. If a caution waves during a green–white–checkered finish, the race is over.