Sodium citrate is the sodium salt of citric acid, used in cooking mainly as an emulsifier that keeps melted cheese smooth and pourable instead of stringy or grainy. A small amount whisked into melted cheese with a little liquid produces a stable, glossy sauce - the basis of modernist "cheese sauce" recipes for dishes like mac and cheese or nachos, and a common trick for a smoother queso or fondue.
Beyond cheese sauces, it is used to buffer pH in some sauces and marinades, and commercially as a stabilizer in dairy products and processed cheese.
Sodium citrate itself is nearly flavorless, with a very mild salty-tart edge; it works by binding fat and water together rather than by adding taste, so it is used in small, precise amounts rather than as a seasoning.