What does the blue flag with a yellow slash in Nascar mean?

Answer 1

It means that you are about to be lapped and need to let the faster car overtake. If a driver fails to let the car pass after three blue flags he is likely to be penalized.

They are also shown at the end of the pitlane if a car is approaching on the track, so that the driver exiting the pits knows to be careful.

Answer 2

It informs a driver that a faster car or cars are approaching and the driver should allow those cars to pass.

Answer 3

Basically, let the faster cars by. This one is optional to a degree.

Answer 4

It means that there are faster cars approaching during the race. Lapped cars should give way to the faster cars.

Answer 5

There has been a crash and to slow down.