[wolfgang42]

Let's say you have an HTML page which links to main.css. Ordinarily, the request goes:

    Client: GET /index.html
    Server: <index.html>
    Client (after parsing): GET /main.css
    Server: <main.css>

Loading the page thus takes 2 round trips, one for the main page and one for the content. (Or more, if you have e. g. includes in the CSS.) Here's what it would look like with HTTP/2 Push:

    Client: GET /index.html
    Server: <index.html>, <main.css> (PUSH)

This only takes 1 round trip; since the server knows that main.css will be required shortly it can preemptively send it. In particular, this might offer a significant speedup for high-latency connections; it also theoretically reduces the need for bundling tools since you can have the server just push all of the individual files.

The obvious problem with this scheme is that if the client already has main.css then it's a waste of bandwidth to send it again. The client can cancel the push, but by the time it finds out about it a bunch of data has already been sent. There is a proposal for 'Cache Digests' which will allow the client to send a Bloom filter of its cache so the server can tell whether or not it has the file already, but as far as I'm aware no major client or server has implemented this yet.