The crude oil product with the biggest demand is petrol, which is made from the naphtha fraction. This fraction contains molecules with 5 to 10 carbons in their chains. As such, it is one of the lighter fractions.
Although around 20% of a barrel of crude oil eventually goes to petrol, there is nowhere near this amount of naphtha in the raw crude. So, the demand is much greater than what is available.
However, there is a surplus of heavier fractions like heavy gas oil, which has very few direct uses. Its molecules have more than 20 carbons in their chains.
The solution is to break up these long chain molecules into the shorter molecules that are useful in petrol. We call this process cracking.
The cracking reaction can be induced using high temperatures in the presence of steam or a catalyst.