The root directory is a directory, where your website's files are located. By default, the root folder of your website is public_html.
To open the root category using SSH, all you need to do is:
Copy the home root path from hPanel Dashboard
Run the following command
Where "/home/u123456789/domains/domain.tld/public_html" is the value of "Home root" which you copied from hPanel.
That's it - after
-bash-4.2$ appeared from the new line, you can start working with your files.