The octane rating of a gasoline is the measure of the anti-knock quality of the fuel. Knock is just that...a hammering sound inside your engine.
Octane became well-known in American popular culture in the mid- and late-sixties, when gasoline companies boasted of "high octane" levels in their gasoline in advertisements. These commercials refer to the octane rating, which is a measure for the anti-knocking properties of gasoline. The octane rating is not directly related to the amount of octane contained in the gasoline.