A histogram is a graph of the image you're working with. It shows how much of the image is pure black, how much is pure white, and how much of it is in between. It helps you see what is the tonal range of your image. You can view the histogram by either going to Window > Histogram or the better one, Image > Adjustments > Levels. What this all tells you is where there are pixels in the image and where there are not and it is either pure white, pure black or somewhere in between. If it is pure black - the image is underexposed (darkness area is to the max - the graph is at a high on the left) and that means there is nothing there but black color; if it is pure white - the image is overexposed (brightness area is to the max - the graph is at a high on the right) and that means there is nothing there but white color.
In the end, all of this information is there to help you balance out your image's color and exposure and adjust it as best as possible.
Hope that helps!
