What is oil refaining?

It's oil refining not refaining. Oil as it comes from the ground is called "crude oil." Crude oil contains many different constituents such as gasoline, motor oil, diesel oil, kerosene, other lubricating oils, propane, etc. Separating these different parts of the oil is called "cracking." Refining is putting crude oil in "cracking towers" where the different constituents are separated.

Cracking towers are like tall tubes standing several stories high. in the bottom of the tube crude oil is heated and turned into vapor. As the heated vapor rises in the tube it get cooler and cooler. As it cools, the different constituents of the vapor turn back into liquid at different temperatures. There are buckets at different levels of the tube (different temperatures) to collect the different liquids.