AMD worked with game developers to come up with a new way to give them low level GPU access in the Xbox One & PS4. AMD is bringing that low level access to PCs with Mantle. They just released a beta driver *to the press only* that includes Mantle support, so expect leaked & public versions very soon.
Most games cannot use Mantle -- BF4's Mantle support is what's mentioned in what I've read so far. An article at guru3d.com says you'll see the biggest benefits right now when the CPU is the main limiting factor, so I'd imagine this is most exciting to anyone playing BF4 on lower end to mid-range systems.
The reason for the early heads up is that from the release notes this may be Very Much a beta release, so anyone planning on trying it might fully back up their systems this weekend.