The source region in weather usually refers to the location where the different types of air masses formed. In general, a source region covers several million square miles. The conditions on the surface of Earth are nearly homogeneous. As an example, the ocean is a common source region for the formation of air masses. Air will remain over the ocean for a period of time sufficient enough to alter the properties of that air parcel.
As an air mass stays over a region, it will acquire the properties of that land or ocean region. For instance, in the example above, the air over the ocean would become moist provided it remain in the same general location for an extended period. Uniformity within the air mass is reached in anywhere from several days to several weeks.