Sub pixels are tiny dots that make up a picture on a screen. They are even smaller than regular pixels, and each one helps create the colors you see on your screen. Full definition
Colors are created by the use of red, green, and blue filters, known as sub pixels, which are then blended by varying degrees to produce different colors. (androidauthority.com)
What this essentially means is that each pixel had a left and right sub pixels. (themrphone.com)
The positive of this technology is that you are able to create a screen that is just as bright as normal screens with one third the amount of sub pixels. (digitaltrends.com)