Nước mắm is Vietnamese fish sauce, a fermented condiment made primarily from fish and salt.
Traditional production layers small fish, often anchovies, with salt in large wooden or earthenware containers. During fermentation, enzymes and microorganisms break down the fish proteins, producing a deeply savory liquid rich in umami. The liquid is drawn off, filtered and graded according to quality and strength.
Fish sauce is used both as a cooking ingredient and as the base of dipping sauces. In Vietnamese cuisine it seasons soups, marinades, stir-fries and dishes such as bún chả. Nước chấm is a broader term for prepared dipping sauces, which may contain fish sauce along with sugar, lime juice, garlic and chili.
Vietnamese fish sauce is not the same as shrimp paste, although both are fermented seafood products. Phú Quốc is especially famous for fish-sauce production and has a protected geographical indication in the European Union. Bottles can differ greatly in concentration, so cooks often dilute or balance them before serving.