Fractional distillation is the separation of a mixture into its component parts, or fractions, separating chemical compounds by their boiling point by heating them to a temperature at which one or more fractions of the compound will vaporize. It uses Distillation to fractionate.
https://www.wikiwand.com/en/Fractional_distillation
For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification.
https://www.wikiwand.com/en/Distillation#/Fractional_distillation
This is used to separate Crude oil into useful liquids that have different boiling points. Petrol and diesel are useful fractions of crude oil.