You might have known already that most monitors produce colours using a combination of only 3 colours. Those three colours are red, green, and blue, and that is what RGB stands for.
In most monitors, each pixel is divided into three subpixels, with each subpixel displaying one of the three colours. Each subpixel can vary its brightness to 256 levels (8-bit), from level 1 being essentially off, and level 256 being essentially turned on at full brightness. By combining the three subpixels with differing brightness, each pixel can then produce 256 x 256 x 256 = 16.7 million colours (24-bit).
I captured the photo below of my Samsung monitor’s screen, and blew it up so that you can actually see the individual subpixels and how they are combined to produce the black colour of the text. It’s something you see everyday but never realise you do. Time for some appreciation, eh?