When crude oil emerges from the furnace it is in two parts - a liquid and vapour. Both are mixtures but the vapour contains a higher proportion of the more volatile (easy to boil) hydrocarbons.
When this vapour mixture meets the first bubble cap tray it condenses to form a liquid mixture. At first, this condensate is the same as the original mixture of hydrocarbons.
However, more hot vapour rises up through the bubble caps and makes some hydrocarbons in this liquid boil. This produces a vapour mixture that has a greater proportion of the more volatile hydrocarbons.