Histogram:
The histogram plots the number of pixels in the image (vertical axis) with a particular brightness value (horizontal axis). Considering the following graph,
Here the vertical axis is showing the no of pixels and horizontal axis is showing the grey-levels of the image. From this histogram of that image it is found that the image contains 75 pixels of 180 grey-level, 100 pixels of 100 grey-levels and 175 pixels of 130 grey-levels.
So in that way, histogram works and it is used. Identifying High and Low Brightness Using Histogram:
Using a histogram it is very easy to identify the high and low brightness of an image. We can know the specific number of pixels containing specific grey-levels. We also know that 0 grey-level for black and 255 grey-level is for white. So if we find maximum pixels containing higher grey-levels then it can be said that the image is of high brightness and if maximum pixels containing lower grey-levels then it can be said that the image is of low brightness.