A sauce is a liquid that has been thickened by a substance known as roux. Roux is a mixture of flour and fat, usually butter, that is cooked over low heat to create a smooth paste. This paste is then added to the liquid and simmered until it thickens. The thickness of the sauce can be adjusted by varying the ratio of roux to liquid. A properly thickened sauce can be used to enhance the flavor and texture of various dishes.