Usually iRacing is not following an specific system of flag signals because there are diverse competitions and series ranging from NASCAR to Indy or FIA road series.
So, I am going to describe few flags we are currently seeing in road series:
Green flag: Start of the race.
Yellow flag: Local caution (that sector).
White flag: Final lap.
Black flag: Return to pits for penalty or disqualification.
Black flag with orange dot: Return to pits to repair your vehicle.
Blue flag: You are being lapped.
Blue flag with yellow stripe: Faster car approaching.
Checkered flag: Session finishing (last corner).
Have a nice and long weekend.