In simplest terms.
Your screen has a pixel resolution say 1920x1080 (so 1920 wide and 1080 high).
Pixel means Picture Element.
A Pixel is really 3 colored LED lights that are close enough together so that they appear as on dot.
Each of the 3 elements in the pixel (Red, Green and Blue) can be set to one of 256 levels, from fully off to fully on.
The Graphics card is responsible for refreshing the monitor a number of times per second (>30) to produce the picture you see.
There is a lot of complex and clever engineering and science that is worth reading about.
Some screens work by actively generating colored lights, others work by blocking off light.
This is highly simplified conceptual description.
Hope it helps.