HTTP/2 (earlier called "HTTP/2.0") is a new version of the HTTP protocol. It is specified in a standard that was published as RFC 7540 in May 2015. As of October 2018 30% of all websites surveyed by W3Techs use HTTP/2.
Apache version 2.4.17 introduced HTTP/2 support. Starting from Apache 2.4.27, the Apache MPM (Multi-Processing Module) prefork no longer supports HTTP/2. Apache Rivet 3.0.0 support threaded MPMs such as mod_mpm_worker and mod_mpm_event. So you can use Apache/mod_http2 (after 2.4.27) and work with Apache Rivet (after 3.0.0).