Retina display has been offered as a feature in apple right from iPhone 4. It has been there for quite a while now. Retina disays have transformed the way how content is displayed on a smartphone.
Retina display basically has got to do with pixels. So in case an ordinary display has a display screen of 4 pixels, a retina display with the same screen size would contain 16 pixels. So in essence it gives more clarity and a sharper image due to a higher number of pixels. There is a perceptible difference for the same content viewed on a retina display as opposed to an ordinary display.