Octane relates to hydrocarbon compounds used in fuel and their ability to burn at the proper moment of ignition, most commonly this relates to 'knocking'. It's kind of a complex subject involving chemistry, automotive engineering, and environmental law, and if you want a more complete answer someone like 'tiptopmanager' on Yedda can find a good reference for you on the net. The levels are the amount of octane compounds the fuel has. Gasoline is blended to have between 84 and 94 octane for passengers cars. 110 octane is for racing fuel. Natural gas has 120 octane. Your car's owner manual will tell you what the proper octane level is for your car. There may also be a label by the fuel fill, under the hood, or in the glove box.