DirectX 12 introduces performance boosts on Windows 8, Xbox One, and Windows Phones; the most important thing to know right out of the gate. Besides catering to mobile games, and requiring less battery juice to be extracted to render particular scenes, Microsoft is also introducing better handling of processing with multiple CPU cores (and avoiding overloading just one).
Basically, DX 12 allows for games on all Microsoft products to perform more efficiently, and specifcally for gaming computers, brings some of the optimizing technology seen on game consoles, in order to lock-in desirable frame rates,
To top it all off: 80% of currently sold PCs will already support DirectX 12; however, games won’t begin to appear wielding the new game development package until the holiday season of 2015.
Via: Direct X Dev Blog