It appears to be where "you" (website visitors) have loaded page tiles. I was able to draw a little picture on the map by zooming in and panning around!
Apparently my 'easy' isn't the same as yours. There's nothing I could find in the code that describes what it is. You'd have to make a pretty big logic leap to figure it out. All you can see in the code (in config.js, not in index.html) is the dataset url (random string) and the description "this website", which tells you very little.
