Roux is a mixture of butter and flour that is used to thicken sauces and soups. The butter provides richness and flavor, while the flour helps to bind the sauce or soup and gives it a smooth texture. Roux can be made with different types of fat (butter, oil, lard) and different types of flour (all-purpose, whole wheat, gluten-free), so it can be tailored to fit the needs of any recipe.