Magento root directory (aka Magento root folder) is first and top-most directory in Magento file-tree hierarchy. This folder contains all Magento files and directories. Typically, this is in the same time, the website root directory as well.
Typical contents of Magento root directory:
You can access Magento root directory via FTP, SSH or Filemanager. For filemanagement over SSH or FTP you can use one of SSH file clients, such as WinSCP or TotalCommander, Doublecommander, Filezilla. In cPanel, you can access webFileManager and navigate to public_html directory.