I couldn't have said it better
The blue flag
A light blue flag, sometimes with a diagonal orange or yellow stripe, informs a driver that a faster car is approaching and that the driver should move aside to allow one or more faster cars to pass. If the flag is waved, it indicates to the driver that the fast car is on its tail. In most series, the blue flag is not mandatory; that is, drivers obey it only as a courtesy to their fellow racers. In Formula One (in which the blue is lighter and does not have a stripe), if the driver about to be lapped ignores three waved blue flags in a row, he is required to make a drive-through penalty. In Champ Car, this can be upgraded. A pair of waving blue flags displayed from the starter's stand, known as the Chief Steward's blue flag, order a driver to give way to faster traffic or be black-flagged.