Both SPDY and HTTP/2 are available to all Cloudflare plans and customers using Universal SSL. While SPDY is enabled by default for all customers using SSL on their site through Cloudflare, HTTP/2 is only enabled by default for customers of the Free and Pro plan and needs to be enabled under the "Network" application for all other customers.
Support for SPDY and HTTP/2 is limited to certain modern versions of Google Chrome, Opera, Firefox, Safari, and Internet Explorer browsers. For more detailed information, take a look at http://caniuse.com/spdy for what browsers support SPDY and at http://caniuse.com/http2 for what browsers support HTTP/2.
All browsers only support SPDY and HTTP/2 over a TLS connection. Therefore HTTPS has to be used to leverage SPDY or HTTP/2.
Note: Google has announced that SPDY support will be deprecated in January 2016, however, SPDY through version 3.1 is supported by Cloudflare right now.
Click here to learn more about HTTP/2 and SPDY.