A Top chart will display top groups for selected timeline. For applications timeline this will be most used applications for any given date range. You can see top 5, 10 or 20 groups for each date range. On all timelines except the Tag timeline you can define Categories (you can do this on Day view). If you have categories set up, you can also see them on top charts.
Example for categories
There are two predefined categories on Documents timeline: Web sites and Documents. To see Top 10 web sites go to Top documents tab (if you don't have one create it) and choose Web sites from the dropdown