The Automatic Certificate Management Environment (ACME) protocol is a communications protocol for automating interactions between certificate authorities and their users' servers, allowing the automated deployment of public key infrastructure at very low cost. It was designed by the Internet Security Research Group (ISRG) for their Let's Encrypt service.
The protocol, based on passing JSON-formatted messages over HTTPS, has been published as an Internet Standard in RFC 8555[4] by its own chartered IETF working group.
The ISRG provides free and open-source reference implementations for ACME: certbot is a Python-based implementation of server certificate management software using the ACME protocol, boulder is a certificate authority implementation, written in Go.
If you want to use lets-encrypt you need this directory.
To use it make a directory in /.well-known.
# cd /srv/www/vhosts/www.example.com/.well-known # mkdir acme-challenge