Demi-glace is a rich, brown sauce used in French cuisine, made by combining equal parts of espagnole sauce and brown stock. It is typically simmered for several hours to develop a deep flavor and thick consistency. This sauce serves as a base for many other sauces and enhances the taste of various dishes. Traditionally, demi-glace is used in meat dishes, providing a savory depth that complements proteins like beef and lamb. Chefs often use it to create sauces for gourmet meals, making it a staple in fine dining and culinary arts.