The Web root is the folder where your website files are put. Typically the folder in which you put your home page.
Common locations are “www”, “htdocs”, “public_html” although for a node site it will be your application folder, something like “nodejsapp” or whatever your choose.
Many systems, especially on shared platforms have both an ftp root (server root) and a Web root which is often a sub folder of the ftp root.
A common setup error is pointing to the ftp root rather than the Web root as all connection checks will fail as the test files will be uploaded to the wrong place.