Retina display is a brand name used by Apple to identify displays that have a pixel density equal to the "resolution" of the human eye retina. Although there is a debate about the actual resolution of the human eye retina, from a developer's point of view what matters is that the pixels per inch (PPI) or pixel density of the screen is high enough to prevent pixilation that is noticeable to the human eye (the amount of horizontal and vertical pixels are actually doubled).
Practically speaking, it's very important to handle the user interface elements in order to avoid a graphic element rendering itself in a pixelated way. If you use standard images on a retina display, they can appear small or blurry; in both cases the user interface will be seriously compromised.
When working with PhoneGap, you are creating a hybrid app based upon standards. For this reason you can handle different pixel densities using CSS media queries and CSS sprites. When using...