Seasoning is a process in which the forms a naturally non-stick protective coating on the surface of the pan. This coating ensures that food does not stick and also protects the pan from corrosion.
The firing consists of applying a thin layer of oil to the surface of the pan, which is then heated to a high temperature. The oil is chemically bonded to the surface of the pan to form a a glossy durable coating that is water and food resistantthat tend to stick.
This is not a one-time process; with each use and subsequent treatment, the the coating and its properties improve.