A histogram is a graph of an image's tonal range. It allows us to see whether or not the image makes use of the entire spectrum of color brightness values, and what changes are needed to improve image quality.
The image below was made using a long-focus lens from quite a distance. It demonstrates aerial perspective: it's hazy because of the large amount of atmospheric particles and dust in the air in-between the lens and the foreground.
Meanwhile, the histogram demonstrates the lack of contrast between extremes of light and dark in the picture below. This image is devoid of vibrant color - it's flat.
To correct the image's tonal range, choose Adjust > Levels.
Move the sliders for the levels of black and white to increase the image's tonal range. The picture will become vibrant once the right levels are found.