When a liquid mixtures is heated in a closed container, the mixture boils and starts forming vapor. The vapor will have different composition compared to the liquid. The vapors are obviously richer in more volatile component. If such vapors are collected and condensed, the condensed liquid will have different composition form original mixture and is richer in high volatile component.
This concept is used in distillation to separate liquid mixtures. The condensed liquid mixture has to be again distilled to get high volatile component in the condensed vapors. Thus by successive distillation, we are able to separate rich volatile component from low volatile component. Instead of repeated or double distillation, fractionating column is used for separation of high volatile components.