AncestryDNA uses Genetic Communities to help find places and populations that might be part of your past. These show up as either migrations or regions with the dotted icon under your ethnicity estimate. Genetic Communities are groups of AncestryDNA members who are connected through DNA most likely because they descend from a population of common ancestors, even if they no longer live where those ancestors once lived.
For example, some Genetic Communities trace their roots back to groups of people who were isolated geographically. Mountains, rivers, lack of roads, or other barriers made it likely that each new generation would marry someone who lived close to home. Others have their roots in groups who typically married others of the same religion or ethnic group. In each case, these groups came to share a significant amount of DNA. Those groups, and their shared past, are reflected in their modern-day descendants who inherited some of that DNA.