In order to deliver resources to browsers more quickly, SPDY employs two mechanisms, Server Push and Server Hint.
Server Push is where the server pushes a resource directly to the client without the client asking for the resource. The server is making an assumption here that pushing the resource is desirable. Pushing a cacheable resource can be risky, as the browser might already have the resource and the push can be redundant.
Server Hint is a mechanism where the server can notify the client of a resource that will be needed before the client can discover it. The server does not send the entire contents of the resource, but rather just the URL as an early part of a response. The client can then validate its cache (potentially even eliminating the need for a GET-if-modified-since), and will formally request the resource only if needed.
Server Hint is implemented using the LINK header with HTTP, and overlaps with existing link prefetching semantics.