This option displays a histogram of the intensity of the image.
It shows the relative number of pixels at each intensity level. If the values are bunched together, the picture lacks contrast; if they are skewed to the left or right, the picture is most likely too dark or too light.
In some cases, equalizing the histogram (using the Equalize button) will improve the appearance. ThumbsPlus will attempt to modify intensities to come as close to even levels of each as possible.
The Stretch button performs contrast stretching on the image, ensuring that the entire intensity range is utilized. This does not change the overall shape of the histogram, but instead spreads it between the limits.
The histogram is most useful for 256-level grayscale and RGB Truecolor images.