In Auto Racing, the Yellow and Red Flag, aka the Surface flag, is usually there to mean that there is debris, fluid, or animals on the track. While FIA-sanctioned championships, IndyCar, and NASCAR uses this flag, NASCAR does not use this flag for the ovals and only for road courses.
The flag is also named the Surface Flag and is displayed stationary at local flag stations to indicate that track conditions have changed due to substances on the track which could reduce grip or cause a car to lose control. Many organizations will display this flag for only two laps, after which the changed surface is considered to merely be "part of the track".
This flag is not in iRacing yet.
FIA-sanctioned championships: Debris, fluid or animals on track.
IndyCar: Debris, fluid or animals on track.
NASCAR (road course only): Debris, fluid or animals on track.