Fish sauce is a seasoning sauce / condiment made from salted fermented fish. Lower quality sauces contain fish that fermented for just weeks while higher end product can take up to 2 years to ferment. Like Seasoning Sauce, fish sauce is mainly used in Southeast Asia and known for its umami flavor.
You can use fish sauce as a condiment by making a dipping sauce with it or as a seasoning sauce by using it while cooking.
People looking for a vegetarian alternative or ones with seafood allergies can substitute soy sauce.