A sausage is meat and spices generally served in a casing of some sort. Sausage is among the oldest of the prepared foods, as it is a byproduct of efficient use of a butchered animal (sausage is often made out of the parts of the animal that are not so appealing to serve on their own, such as organ meats). Additionally, some forms of sausage serve to preserve meats for long-term storage, over journeys of medieval winters.
Sausage is a traditional "roughing it" food in the SCA, as it's inexpensive, easy to transport and store, and relatively tasty and filling. A large summer sausage and a decent-sized loaf of bread, and you can feed yourself for a weekend!