The histogram is a powerful feature that can give a photographer a lot of information if that person knows how to read it and what it means. The histogram is just a graph that on one side shows the colors from dark to light and on the other side is the amount of pixels that has that shade.

So if there is a big bunch of data to the left of the histogram the picture has a lot of dark pixels (and might be underexposed). If they are all bunched on the right side the picture has a lot of light colored pixels in it and might be overexposed.

It is up to the photographer to decide when looking at the histogram is what is intended. If you are shooting a person against a white wall then you would expect a big group of the data on the histogram to be to the right side.