From the site:
What is this map? Read more about this project.
When an unregistered user edits Wikipedia, he or she is identified by his or her IP address. These IP addresses are translated to users’ approximate geographic location. Unregistered users only make a fraction of total edits — only 15% of the contributions to English Wikipedia are from unregistered users. Edits by registered users do not have associated IP information, so the map actually represents only a small portion of the total edit activity on Wikipedia.