A histogram is a graph indicating the image's brightness. The horizontal axis indicates the brightness level (darker on the left and brighter on the right), while the vertical axis indicates how many pixels exist for each brightness level. The more pixels there are toward the left, the darker the image. And the more pixels there are toward the right, the brighter the image. If there are too many pixels on the left, the shadow portions will lose detail. And if there are too many pixels on the right, the highlights will be washed out. The tones in-between will be reproduced.
By checking the image and its histogram, you can see the exposure level pattern and the overall tone reproduction condition.
