French for "red," a roux is a thickener for sauces and soups that combines equal parts flour and butter.
Roux ("roo") is used to thicken sauces and soups. Pre-cooking flour allows the starch granules to swell and absorb moisture, and lets you thicken a sauce base without the flour clumping or forming lumps. Rouxs are also used to deepen the flavor of a sauce: browning the flour gives it a nutty, toasted flavor.
Colors of roux
Roux can be cooked to white, blond, brown and dark brown stages. The color depends on the temperature and time that you take to cook the fat-flour mixture. None of the types of roux are necessarily better or worse than any other; the color you choose will depend on the flavor and color you would like your roux to give your sauce or soup. For an in-depth description and photos of different types of roux, see our All About Roux article.