I'm using stablehost and have been searching cPanel for a while trying to find my directory. I'm on macOS.
The webroot is the location on the webserver where all the files of the website are. I.e., if you surf to http://example.com/foo-bar.txt, you would expect to find the file foo-bar.txt in the webroot of your website.
Welcome to the Let's Encrypt Community, Joe 
Your webroot is usually public_html with cPanel, unless you are managing multiple domain names under one hosting account, in which case these "addon domains" usually have their own subfolders under public_html with the primary domain name having its content directly in public_html.