What is a Document Root folder?
The document root folder is the main folder which has all the files of your website. The root folder of your primary domain is public_html folder. The root folders of your subdomains and addon domains will be present inside public_html folder.
Note: Addondomains can be viewed just like the subdomains as primarydomain.com/addondomain. To avoid this, you can make the document root folder of addon domains outside the public_html folder like public_html_addondomain.com, etc.