The maps shows air traffic: The part of China that is not yellow on the map in uninhabited, there are no airports so also no air traffic. Just like the Sahara. "The eastern coastal part of China accounts for the majority of travel, with 70% of 2013's 754 million passengers flying in the region. "
Besides the clue Flyboy is giving already, China's airspace is also heavily restricted. Most of it is for military use only, leaving only small corridors for civilian traffic. This can also seen on the image, as the areas that are yellow, are all direct lines.