An oven is an enclosed structure usually with a door in which things are baked. In period, ovens could be made of anything from a hollowed pile of dirt to a brick-and-mortar oven. Individual household ovens have been in use since 3200 BC in the ancient Indus Valley  and by ancient Greeks but many cities in period had a professional baker who would run a public oven. This public oven was utilized by households that didn't want to go through the hassle or expense of building, maintaining and firing up an oven every day. Instead, they would bring their unbaked breads to the baker to bake.
The slashes commonly seen on the tops of specialty breads in modern day are said to be a relic of the public bakery, in which households would mark their loaves with individualized slashes to ensure they picked up the bread they dropped off.