Metal is a new proprietary API designed by Apple to provide low-level, low-CPU overhead access to graphics hardware. Only iOS A7 (and presumably onward) devices are supported in iOS 8 as I'd imagine that a new, quite specific, driver model is required to achieve this.
Epic demoed Unreal 4 running on Metal because for UE4 games on iOS, Metal should enable games to make better use of the GPU so they can run faster and/or render greater detail. In many ways it is much more important to have low driver overhead on mobile because the CPUs are so comparatively anaemic when compared to desktop machines or even laptops. Not to mention the vastly larger market.
There's no Mac version, OS X still ships with OpenGL 4.1 as the only graphics API. However, AMD's Mantle on Windows demonstrates that it should be possible to develop a similar API on desktop. The trick will be supporting more than one specific GPU architecture. You'd still need some abstraction as other posters mention - but the big wins are to be found by realigning API design toward the workings of modern GPUs and pushing much more responsibility for correctness onto the graphics programmer and away from the driver programmer. That could eliminate a lot of the state validation and error checking that an API like OpenGL has to perform, which is potentially just wasted time. Microsoft announced a similar approach for Direct3D 12 and hopefully Khronos follow suit with the OpenGL 5 specification so these improvements should eventually arrive on desktop platforms in some form.
