HTTP/2 is a successor to HTTP/1.1 and is based on the SPDY protocol developed by Google. The SPDY protocol has been deprecated and browsers are removing its support.
The main advantage from clients perspective are faster page loads - the new protocol has lower latency and enables multiple HTTP requests via one TCP connection.
HTTP/2 requires encrypted communication. The RFC does not specify it, but there is no unencrypted implementation.
Debian Stretch, released in summer 2017, has native support for Nginx as well as Apache. The same applies for Ubuntu 18.04 and newer.