Histogram is the best tool to understand if a photo is correctly exposed or not, but often is the most ignored tool by photographers or aspiring photographers. Yeah, in most of the cases you can see with your eyes if the photo is well exposed or not, but the histogram can tell you quickly if there's something wrong that you might have missed on a quick glance. And there are situations where you can't simply trust what you see on the screen of your camera, for example on a very sunny day in summer, where is quite difficult to see perfectly on the screen.
In this video by John Greengo made for CreativeLive it simply explains how to read the histograms you can find in any DSLR or MILC.

(via PetaPixel)