Metal in iOS 8: explained
 

Metal. If the name sounds hardcore, it's because it's a hardcore improvement to the way games will be able to perform on iOS 8. Metal represents a much more no-nonsense approach to getting the most out of the Apple A7's gaming performance, assuring users of the iPhone 5S, iPad Air and iPad mini with Retina display that their devices will continue to be top-notch game systems come this fall. Right now in iOS 7 software called OpenGL ES sits in between the game and the core hardware that runs it, translating function calls into graphics commands that are sent to the hardware. It's a lot of overhead. And iOS 8 is getting rid of a lot of it. A nice overview of Apple's Metal.

http://osnews.com/story/27809/Metal_in_iOS_8_explained