This is an ACME Certificate Authority running Boulder.
This is a programmatic endpoint, an API for a computer to talk to. You should probably be using a specialized client to utilize the service, and not your web browser. See https://letsencrypt.org/docs for help.
If you're trying to use this service, note that the starting point, the directory, is available at this URL: https://acme-v02.api.letsencrypt.org/directory.