These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together.
Butane has four carbon atoms chained together.
It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio. During WWI, it was discovered that you can add a chemical called tetraethyl lead to gasoline and significantly improve its octane rating. Cheaper grades of gasoline could be made usable by adding this chemical.