Images of such high resolution that the human retina can no longer resolve individual pixels. It's an Apple buzzword for High DPI. Back in the day Apple retina displays were way higher resolution than anything from their competitors. They even invested heavily in the manufacturing equipment themselves to push the technology forward, but nowadays screens with those sorts of resolutions are commodity.
It's one of those cases where Apple identifies a technological gap of a few years they can open up between them and the rest of the pack and then invests in stealing a march on everybody else. Eventually the others catch up, but meanwhile if you want the best in that category only Apple has it. Right now in screen tech it's wide colour gamut and 5K desktop displays.
Retina in this sense is an Apple marketing buzzword for high DPI anything. Back in the day they used to have higher DPI items, but today they still put 1080p displays on their 5.5" phones while most other flagships use 1440p in the same size display (and are thus much higher DPI).
You are right but I wonder if it is any improvement at this point. Writing this on a iPhone 6s Plus and I can't possibly see individual pixels. What is the benefit of going higher?
This is a niche answer, but you can put them in a VR headset with lenses that spread out the screen over a much larger fraction of your visual field without looking too pixel-y.
Hold your 6S Plus up next to a Note 7 viewing the same photograph or high resolution video. If you don't see a large difference, head immediately to the optometrist.
