see examples of webroot authentication plugin at Using the webroot domain verification method and https://community.centminmod.com/threads/letsencrypt-free-ssl-certificates-with-web-root-authentication-method.4635/
official definition
Authenticator plugin that performs SimpleHTTP challenge by saving
necessary validation resources to appropriate paths on the file
system. It expects that there is some other HTTP server configured
to serve all files under specified web root
it was born out of the simplefs plugin SimpleFS plugin (fixes #742) by kuba · Pull Request #757 · certbot/certbot · GitHub which was later renamed to webroot authentication
laymen terms, webroot authentication is an alternate way to obtain letsencrypt ssl certificates and pass the SimpleHTTP challenge by following these steps
- create a HTTPS base site before hand using self signed ssl certificate on apache or nginx - this site will have a public web root. This site domain also needs valid working DNS pointing to the server IP
- run letsencrypt webroot authentication method and pass your email address AND that site's public web root path to the command you run - this will perform automatically the the .well-known uri creation on the defined web root validating the domain you want the ssl certificate for