Resolution is important, but the size of your screen makes just as much of a difference. 720p looks great on a 5-inch screen, but it will be much less sharp on a 50-inch screen. This is what we refer to as pixel density, or pixels per inch (PPI).
"Retina" is Apple's term for a screen that has such good pixel density that you can't actually see the pixels on the screen, when viewed from a normal distance. Note that this can mean different things depending on the device; it's a marketing term, not a scientific definition.
Some of this you may already know. But if you're trying to figure out whether a screen counts as "retina" or not, check out this calculator. You plug in the resolution and the size, and it will tell you how far away you need to sit in order to have that retina experience.
Overall it's an informative video and a cool tool, even if you're familiar with the concept. If you have less tech-savvy friends that still don't really know what retina means, it's a quick lesson that will get them up to speed.