HTTP/2 is the first major HTTP protocol update since 1997. The main goal of HTTP/2 is to decrease latency, reduce total number of TCP connections thus improve page load speed in web browsers.
HTTP/2 is backwards-compatible with HTTP/1.1, all application semantics of HTTP are the same, except the way of transmitting data via TCP connection. HTTP/2 leverages multiplexing and allows asynchronous (parallel) requests and provides data compression of HTTP headers. Also, the server push method in HTTP/2 allows server to send multiple responses for a single request. HTTP/2 support is not included in Ubuntu 16.04, neither with nginx nor with Apache because this feature is considered as experimental by the Apache httpd upstream project, so it is not possible to enable it on an Ubuntu 14.04 VPS using the a2enmod command.