The root directory of your website is the content that loads when visitors access your domain name in a Web browser. The most obvious consequence of this is that you need to put your "index file" in your website's root directory for visitors to see your Website at all (more info).
Website-related applications might also need to know your website's root directory.
Your website's root directory depends on whether the domain name is your primary domain name or another type (more info).
Your primary domain name's root directory is
Secondary and subdomain names' root directories were specified when they were set up. You can find them listed in Plesk.
In the section for each domain name, the root directory displays in the Website at field. By default, the root directory is the domain name itself followed by a forward slash, e.g. subdomain.coolexample.com/