A document root (or web root) is the folder from which a site loads.
By default the master/main domain of your hosting is mapped to the public_html directory, so public_html is the document root of your master/main domain. Single domain hosting accounts (Starter and Essentials hosting packages) operate in this manner.
If you have a Unlimited, Multisite or Business hosting account, these allow for add on domains to be created. You can find out how to create an add on domain here.
By default, unless you manually specify otherwise, an add on domain's document root will be a folder with the same name as the domain.
You can use the "Domains" section of your cPanel to see which domains have which folders set as their document roots (you can also click on the folder listed in the "document root" column to open the file manager and access the folder set as the document root):
