A salty condiment used in many Asian countries
Fish Sauce is an essential ingredient in south east Asian cuisine, where it is used as a replacement for salt. It is particularly associated with Thai cooking, where it is known as nam pla, but it also turns up in in other countries in the area.
It is made by fermenting down small fish and drawing of the pungent liquid that results. It's used both as a condiment in dipping sauces and as a cooking ingredient, and when cooked it's pungency is tempered a little.
Once an exotic ingredient, it is widely available these days in larger supermarkets as well as Asian shops.
If, however, you can't get hold of any, or if you find it's fishiness offputting, then a light soy sauce makes a good substitute.