The Disk Usage Viewer shows all directories starting at the Account Root level. These directories are for your use / access only and also used by cPanel. Best not to make any changes unless you know what you are doing.
The exception is the "public_html" directory, also known as Document Root. Contained within this directory are sub-directories & files that are publicly available to your site Visitors. These would usually be manually created by yourself or from scripts you've installed, such as your Home Page and perhaps a Forum script.
For more information you can read our Tutorial on Know Your Roots - link opens a new Browser window.