Retina Display - The name given by Apple to the high-resolution screen technology introduced on the iPhone 4 in June 2010.

Retina Display is designed to smooth the jagged edges of pixels are provide a higher-quality image than previously available on mobile devices. Apple claims that its resolution is so good that it makes it impossible to distinguish individual pixels. The effects of the display technology are noticeable in many uses, but especially in text, where font edges or curves are substantially smoother than on previous display technologies.

Enabling your app's images for Retina Display is very simple:

Instead of entering in the standard size image in the editors such as a 1024x448, just double the required size to 2048x896.