The Smallest Building Block
If you look very closely at a screen, especially through a magnifying glass or a drop of water, you can sometimes see what looks like a fine grid of glowing dots. Each one of those dots is a pixel. The word itself is short for picture element. A pixel is the smallest controllable point of color a screen or digital image can show.
Many Pixels, One Picture
A single pixel on its own is just a tiny patch of one color. The picture you see is built when millions of these pixels sit side by side, each set to a slightly different shade. Your eyes blend them together at a distance, and what looks up close like a mosaic becomes a smooth photo or a sharp line of text. Both digital cameras and digital displays are organized around this same idea of a grid of points.
Resolution and Sharpness
The number of pixels in an image or on a screen is called its resolution, and it is usually written as two numbers, like 1920 by 1080. The first is how many pixels wide the picture is, the second is how many tall. The more pixels there are in the same space, the finer the detail. The same idea sits behind the word megapixel for cameras: one megapixel is one million pixels in the sensor that captures the photo.
Source
This article was written using information from Wikipedia.