Like AMD's Mantle API, it promises reduced CPU overhead and lower-level access to graphics hardware. But DirectX 12 won't be limited to one vendor's hardware.
Mantle will work on this hardware as well, and won't be limited to one vendor's operating system. From AMD's Mantle FAQ
In the future, you're going to get computers as prizes in breakfast cereals. You'll throw them out because your house will be littered with them.