When you see au jus on a menu, you'll know the dish it's describing comes in a very light sauce. Various kinds of meat are often served au jus in French cuisine.
The phrase au jus literally means "with the juice" in French. In cooking and dining, it refers specifically to the "juices" that are released when you cook a piece of meat. Traditionally, serving something au jus means spooning these juices over the meat before serving, although in American cooking au jus can also mean a delicate sauce.