A Picture Made of Dots
Up close, every screen reveals its secret. What looks from a normal viewing distance like one smooth, glowing image is really a grid of millions of tiny squares of light called pixels. Each pixel is a single point that the screen can light up in a particular color. Put millions of them side by side, give each one its own job, and the eye blends them together into a continuous picture.
Electricity Tells Each Pixel What to Do
Inside the screen, every pixel is connected to a network of tiny wires that carry electrical signals. For each pixel, the signal carries two pieces of information: what color it should display and how bright it should be. To make any color, the screen does not actually create that color directly. Each pixel is built from three smaller sub-pixels, one red, one green, and one blue. By mixing the brightness of those three at different levels, a single pixel can produce any color the eye can see.
A Picture Drawn Many Times a Second
A still picture would be one thing, but screens also show motion. They do this by redrawing the entire grid of pixels many times every second. Each new frame is slightly different from the one before, and because our eyes blend rapid changes together, the result looks like smooth movement rather than a flickering slideshow. The faster the screen can refresh, the smoother fast action appears.
Different Screens, Same Idea
Most modern screens come in one of a few main types. LCD screens use a backlight that shines through a layer of liquid crystals, which act like tiny shutters in front of color filters to control each pixel. OLED screens go further by having each pixel produce its own light, with no separate backlight at all. The technology underneath is different, but the principle is the same: take a vast grid of pixels, control each one with electrical signals, and the picture appears.
Source
This article was written using information from Wikipedia.