The root directory is the folder that is located in the "root" of a directory structure.
It is comparable to a tree:
A tree has a trunk (= the root directory) and is the basis for branches (= the subfolders) or leaves (= the files in the folders).
The following screenshot shows this division:
When a website is called up, the world4you web server checks whether there is an index.html or index.php file in the root directory and displays it.
The root directory is therefore the entry point of your website. Below it are subfolders and files.
As another example, we are in the "bilder" folder in the screenshot below. There you can see that "bilder" is a subfolder (= "branch") of the root directory (= "root"). This subfolder in turn contains files (= "leaves").
You can manage the root directory, subfolders and files at any time via your FTP access.
By the way: configuration files (e.g. .htaccess) or search engine-relevant files (e.g. sitemap.xml, robots.txt) are also stored in the root directory.