The term "exurban migration" is the same as what is sometimes called "reverse migration." This is migration in which, instead of moving from more rural areas to an urban core, businesses and residents move out of a central city to communities that are outside the central city's ring of suburbs.
In the past, the trend was for businesses and people to centralize. Big cities grew up because of this trend. Exurbs (wealthy area beyond the suburbs) tended to empty and become, at most, bedroom communities or commuter towns. However, in the past few decades (in the United States, at least) this trend has reversed. The idea of living in a central city or of having a business located in such an area has become less popular. As this has happened, suburbs and even exurbs have boomed.
Exurban migration, then, is this "reverse" movement out of the cities and into relatively distant, exurban communities.
The word "exurban" refers to "out of cities". You can see this if you break the word down.
Ex - out of, as in, external (outside of yourself)
Urban - refers to cities. urban communities = communities within a city
So, "exurban" refers to "out of cities". Migration refers to people moving from one place to another. "Exurban migration", then, would refer to people moving out of the cities to more rural areas. This is opposite the usual trend - usually people move into cities, rather than out of them. However, it has become increasingly popular to work in the city and live in a nice "country home" located just beyond the city's outskirts. This is particularly common with the wealthier class. This does tend to create a social problem, however, if it occurs as a trend in a particular city. Wealthier families moving out of the city means the center of the city is more prone to poverty, crime and poor housing opportunities.