MICHAEL ARCHAMBAULT, PETAPIXEL:
"Image histograms are useful tools that can be used to help properly gauge the exposure and contrast within a scene during shooting or to balance the tonality of a photograph in post-production. These days, histogram graphs can be found almost everywhere, with more modern digital cameras including live displays during composition.... An image histogram is a graph that plots the tonal distribution within a photograph. In much simpler terms, it can help us to determine whether or not an image is correctly exposed based on the amount of light and dark pixels."
Read more to learn how to read and interpret image histograms, here.