Why do you care what is a histogram ?Who cares what is a histogram? For much of the digital photography many photographers do, we will not need to look at a camera's histogram. There are certain times when it is an extremely valuable tool for getting our exposure right on the money. A basic understanding of what they are and how histograms are used will help you become a better photographer. The simplified definition of a histogram is a graph that shows the range of exposure form the darkest shadow ares to the brightest highlight areas in our photograph.
Digital camera meters and their built in processing software do a great job of estimating the correct exposure most the time. When you have an unusual subject or unusual lighting situation, these graphs are very valuable. The two warning situations occur when the dark area underneath the graph more than just barely touches either side of the graph. That is the most important thing to know. You can see the three examples with the snapshots of my dog "Mocha" sleeping on my office floor. The histograms for each photo are directly below them. In the left photo, it is clearly under exposed because the graph underneath that dark photo show a significant amount of the black area is touching the left side of the graph. If we kept the exposure at this level, we would lose details in the shadow areas and brightening when we do our photo editing would reveal lack of detail and too much digital noise in the edited photo. The middle photo is properly exposed. The photo on the right is overexposed. It looks "about right" but if we look at the histogram we can see that the graph has an area that is against the right side of the graph. In this situation we have lost details in the highlight areas that are lost forever and can never be fixed with editing afterward.
Most of the tones in this photograph are dark, so the histogram will have most of the area of the photograph on the left side of the graph. It is not under exposed. There is no black area in the graph that is touching the left side. The best way to learn more about them is to look at your histograms after taking several different photos with different subjects. You can have some fun by guessing ahead of time what your graph will look like. you will be surprised how much this will help you in those situations that need extra attention in getting the correct exposure for your digital photos. Back from What is a Histogram to Photography Definitions
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This photo of Mocha is perfectly exposed. This is the biggest photo mistake that many photographers, even many un-educated pros make when it comes to evaluating histograms.