I first learned of this phenomenon about 10 years ago on my first trip to China.  Imagine the population of Russia walking around China looking for work.  From Yao Lu, a sociologist at Columbia:

The conventional term for internal migrants in China is “floating population,” a phrase that describes unprecedented migrant flows, moving from inland villages in the underdeveloped central and western regions to China’s costal cities, searching for work. According to recent estimates, the total number of migrant workers is more than 150 million, perhaps the largest movement of labor in human history. The floating population is a major force fueling the country’s rapid economic growth. Having made China one of the largest export economies in the world, migrant workers have also become visible to those outside China.

[...]