What is this?
The /.well-known/ path is a standardised place on your web server (RFC 8615)
where machines look for expected config and metadata files. Examples: security.txt, mta-sts.txt, openid-configuration, apple-app-site-association.
This tool takes a domain and checks the usual list of well-known paths - which are reachable, which are missing, which respond with a wrong content type or a redirect.
When do I need it?
As an audit tool before pushing a domain into production, or as a sanity check after a migration. Also handy when you want to see which paths your competitors or vendors already serve - it tells you something about their respective security maturity.