An LED Screen is like a giant television, but with one fundamental difference: instead of the picture being beamed from a cathode ray tube, each pixel is made up of a cluster of tiny LEDs (like the ones you find in modern bike lights, but brighter).

Each cluster on an LED screen has a red, green and blue LED, which light up accordingly to create the correct color.  If you look very closely at your television set then you can see individual pixels.  If you look very closely at an LED screen, all you see is lots of very bright little lights, because the pixels are much bigger, but when you get further away the picture becomes clearer.