Plain English Meaning
A specific type of aviation fuel for piston-engine aircraft. The number 80 tells you how much engine pressure it can handle before causing knocking. It is colored red so pilots can recognize it on sight.
Full Definition
A grade of leaded aviation gasoline with a minimum lean-mixture octane rating of 80, dyed red, intended for low-compression piston aircraft engines that do not require higher-octane fuels. It contains a small amount of tetraethyl lead and is sometimes labeled 80/87, where the second number reflects performance under rich mixture conditions.