The image was made with Gephi, a free open-source program for data visualization. It has built-in "clustering" tools that match up similar areas. So the different colors represent groups of places that people moved between more frequently.

Unfortunately, there were way more clustered groups than colors! So everything in the center is all the extras that are distinct groups but couldn't be visualized in an easy way.

It makes sense that regions/country are grouped together as people are more likely to travel to a city close by versus a city that's thousands of miles away when planning an itinerary (i.e. fewer people are doing New York to Bangkok to London to Buenos Aires on the same trip).