We began by talking about Stainless Steel. Stainless Steel is normal steel, plus Chromium (which is an element) and a few other substances. When Chromium is exposed to Water, it reacts with the Oxygen in Water and forms a very thin layer of Chromium Oxide.
This layer protects the rest of the steel from rusting. This module hasn't directly explain how Chromium Oxide is formed! But, you will learn about these things in later classes. Do not forget the concept of valency until then!