First, the therm raft foundation is explaining consistence of a raft of reinforced concrete under the whole of a building. This is a type of foundation that is described as a raft in the sense that the concrete raft is cast on the surface of the ground which supports it, (as water does a raft) and the foundation is not fixed by foundations carried down into the subsoil. This video and the link bellow will explain it better and mote visual.

Source.

...