It is a graphical summary of the overall image exposure.
you can use it then to lighten/darken as required.

If you use the histogram feature on your camera (if it has one) then you will learn more about exposure and how your camera handles it.

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In addition, when shooting, you can choose to show the histogram on this disply on your camera, it will give you the ability to understand the exposure opn your shot and make adjustments.

The histogram itself is just a graph of color tone (or light) in a picture. The darker aspects are on the left, lighter on the right.

In a photo editing program you can use the slider on the histogram to brighten/darken.