Thanks for the question. WP is simply shorthand for "WordPress." Sorry if it was confusing you.
The easiest way to describe "root" in relation to the wp-config file is simply to state that it is the location where your installation files for WordPress are located. By definition, your default Web ROOT, is located in the /public_html/ folder (open up your File manager and look for the Public_html folder).
Therefore, if you installed WordPress to your main or primary domain URL, then the files will be located in the /public_html/ folder. This folder is the ROOT (or root folder/directory) of your installation.
In contrast, If you installed your WordPress website into a folder located inside the public_html folder, then the hierarchy would look something like this:
This means that "wordpress-install-folder" is a folder/directory located inside of the public_html folder. If you installed it into this folder, then the "wordpress-install-folder" is now the ROOT folder of your WordPress installation.
This explanation is used to illustrate WHERE the files of the WordPress installation would be located. By default, the WP-CONFIG.PHP file is located in the root directory of the WordPress installation.
I hope this helps to clarify the issue. If you want more information on the term, you can look it up here