After reading and playing for some days with HTTP/2 this is a summary of my understanding at a very high level.
HTTP/2 is all about reducing the latency accessing web applications. It maintains the semantics (GET, POST... methods, headers, content) and url schemes of existing HTTP and it is based on the improvements proposed by Google as part of his SPDY protocol that is finally replaced by HTTP/2.
The three most important changes introduced in HTTP/2 in my opinion are:
1) Reduced overhead of HTTP Headers by using binary fields and header compression (HPACK).
2) Ability to use a single TCP connection for multiple HTTP Requests without any type of first in line blocking (Responses can be sent in different order than requests).
3) Support for pushing contents from server to client without previous request (for example the server could send some images that the browser will need when it receives the request for the HTML file referencing those images).
The most controversial "feature" of HTTP/2 was making TLS mandatory. At the end the requirement was relaxed but some browsers (firefox) plan to make it mandatory anyways.
Most of the relevant browsers (at least chrome and firefox and some versions of IE) already include support for HTTP 2 as well as the most popular opensource servers (nginx and Apache). So you should be able to take advantage of the new version of the protocol right now.
The HTTP/2 support is negotiated using the same HTTP Upgrade mechanism used for websockets and should be transparent for users and elements in the middle (proxies),
Application developers should benefit for free automatically without any change in their apps but they can get even more benefits with some extra changes:
* Some tricks that are in use today like spriting or inlining resources are not needed any more. So they can simplify the build/deploy pipeline.
* Server push could be automatic in some cases but in general will require developers to declare the resources to be pushed in each request. This feature requires support in the web framework being used.