Octane is a measure of the relatives ratios of the octane molecule with some other (I can never remember which). 100 octane means the fuel acts in your engine the same as fuel made from 100% octane molecule. 0 octane means it acts like 100% the other, 90 octane mean it acts like a mix of 90% octane and 10% the other.

You can go below 0 or above 100 if you find some other molecule/mix that acts worse/better, though how you extrapolate beyond the 0-100 range is subject to debate, confusion and deception. Ethanol is clearly better than pure octane, though how much depends on how you choose to extend the scale.