Fish sauce is a Southeast Asian staple, made by fermenting small, whole fish in brine, drawing off the liquid, and steeping that in the sunlight before it is bottled. In Vietnam, it is called nuoc nam, in Thailand, nam pla, and in the Philippines, patis. It has a very pungent, salty taste, and is an essential ingredient in many dishes in these and neighboring countries.
Fish sauce is also used as a condiment, and plays a broad role similar to that of soy sauce in Chinese and Japanese kitchens. Different varieties of fish sauce are available in most Asian markets, some well-stocked supermarkets, as well as online.
For some people who did not grow up consuming fish sauce, it is an acquired taste.