Crude oil is a mixture of different chemical called hydrocarbons. The boiling oil turns into a mixture of gases in the column. The gases flow up the column which is hottest at the bottom and cooler at the top. The gases cool down as they go up the column until they condense (turn back into liquid again). The separated liquids and gases, after cleaning and further processing, are used to make many products.
The heavier liquids are in less demand from customers so are turned into lighter products that are in demand. One of the processes is called catalytic cracking. It breaks down some of the heavy liquids from the distillation column.
The heavy liquids are changed into simple and more useful liquids and gases. Cracking is just one of many chemical changes in an oil refinery.