Glossary:
Root folder
The root folder or root directory is the folder at the top of a hierarchy (ie the highest or peak).
For a website the root folder usually refers to the public_html folder or www folder of a given website. This is the folder that contains the index.html home page, htaccess, robots.txt and holds all other folders relating to the site.
Note that this will not be the root folder of the web server, that would refer to C:\\ however this would only be accessible to the web server admin.