This window allows you to monitor how many avatars are present in regions of your chosing.
To add to the list of regions to be monitored, open World Map, use it to find the region of interest, and then click on Track Region. From this point, the viewer will keep track of avatar counts on each region you added.
Open the tracker via the top menu, World → Region Tracker. A window is opened, like the one shown here. The window lists:
Click the circular arrow to force a refresh of the avatar counts. The trashcan icon will remove the currently selected region from the tracker, while the last icon will display the selected region in the world map.