Tree statistics are only available for the UrlPath field, the FtpPath field for FTP sites or the LogicalPath field for the Root folder or the All sites modes.
Once enabled statistics are calculated for every folder, sub-folder and page and displayed as a tree allowing to quickly understand which section of the web site is more visited.
It is then very easy to click on a folder ∈ link to only see requests to this particular folder. This will generate a filter in the form UrlPath LIKE '/folder/%' You can also click on the ∉ link to display all request outside this specific folder.