Until today, I finally discovered how histogram can be used to tell me how each taken picture is exposed. But as far as I am concern, we should still use one as a guide and not the definitive way we compose and portray the outcome of the intended picture!
A histogram is a graph counting how many pixels are at each level between black and white.
Black is on the left. White is on the right. This applies to the RGB histogram that comes available with each of our picture.
The height of the graph at each point depends on how many pixels are that bright.
Lighter images move the graph to the right. Darker ones move it to the left. Easy!
A good image often, but not always, has a histogram spread all over.