The document root is the folder where the website files for a domain name, your website, are stored. Since cPanel allows for multiple domain names (add-on domains AND subdomains), you need to have a unique folder for each domain.
Your primary domain is rooted in the public_html folder. Your add-on and subdomain names will be rooted to their own folders inside public_html. After you create an add-on domain or subdomain, you can find the document root inside cPanel by clicking on the Addon Domains or Subdomains icon.
Tip! You can click the document root name in cPanel to go to the correct folder in the File Manager.