Foundation is the basic building block of your prison. When selected and given an area a Foundation creates 4 walls, a ceiling, and light fixtures within the room.
At present foundations appear to be the only way to get a roof on an area, using brick walls only creates a boxed in area without a roof (which is useful for Yards).
Foundations also require that there be some kind of entrance (a door of any kind from the objects tab) to be completed. I have big dick