If you've ever eaten at a Vietnamese restaurant and ordered the spring rolls, you probably got a small dish of this style of sauce for dipping. But where many fish sauces are chock full of nasty preservatives and whatnots, this one has only two ingredients: anchovies and salt.
It's very similar to our garum colatura, the anchovy sauce from Naples, though the aging and fermentation are different. If you cook Southeast Asian food you'll find lots of uses for it. In Western dishes you can add a dash while sautéing onions or to the base of any sauce to give the flavor a kick. The same goes for soup. I also think it’s great for meat marinades and sauces, but keep the brakes on—a little goes a long way.