Retina display refers to a high resolution digital display.
Apple quote: Text in books, web pages, and email is crisp at any size. Images in movies and photos are stunning at almost any angle. That’s because the Retina display’s pixel density is so high, your eye is unable to distinguish individual pixels.
To understand if the answer is truly a retina display you need to understand visual acuity. However there is no question that Apple retina display are high resolution.
See also The reference pixel on W3C site
 Is it needed?
Users will differ on whether it is useful, important, or perhaps absolutely necessary. Some claim that they get headaches on any other type of display while others say they can't tell much difference in actual use.