If the histogram is distributed on the horizontal axis, then the image in the photo is bright and contrast enough. If the color curves shown on the histogram have peaks and are concentrated at one horizontal area (for example, in the middle), the image at the photo needs to be edited.
If the histogram is moved to the left, then we see the dark low contrast image. If the histogram is moved to the right, it is light low contrast image. The histogram with the peak in the center means that the image has not enough contrast and most of pixels have medium brightness. If the histogram has peaks in the left (in the dark area) and in the right (in the light area), and small amounts in the middle (in medium hues), then we see the contrast image.
If the image in the photo is not saturated enough in terms of colors, and not sharp enough, you can change Brightness and Contrast parameters, as it is said in Histogram panel description section. (For example, for decreasing the contrast you can make dark areas lighter and light area darker at your histogram).
Making changes of brightness and contrast at the histogram graph, you can at any time reset the approximate values. In order to do that, click the right button of the mouse, and press at the appeared icon Reset aligning.
Let us build the histogram for our example.
In cells of 4х4 pixels matrix brightness values for each pixel are written. In this case brightness values lie in the 0 to 9 range.
In the matrix there are 2 pixels with the brightness equaling to 1; 5 pixels with the brightness equaling to 2, and so on. We mark at the horizontal axis brightness values, and at the vertical axis — amount of pixels having this brightness. Then we unite the received points and get the histogram.