"Retina Display"TM is a brand name used by Apple for LCD which they claim have a high enough pixel density that the human eye is unable to notice pixelation at a typical viewing distance.
source : Wikipedia
In other words, it is all about the screen of the iPads (of 3rd and 4th generation) that is also used in the latest iPhone models with even higher pixel density as one looks at the mobile's screen from a closer distance.
As I guess you will already have witnessed, the images on your tablet's screen appear almost as they were printed on a glossy hard-copy magazine page.