A folder root is the highest folder in a hierarchy of folders.
Let’s say for example that all your website files reside on your computer in a folder named domain. That makes the domain folder, the root folder or the root of your website.
When somebody is telling you to copy a file to the root of your website, that means that you should copy the files in the domain folder.
On a server usually the root of your website is called public_html and all the website files will reside there, which makes public_html your root folder.