Pretty much like the title says.
In VFB and also in Photoshop and I geuss any other graphics program, you can view the histogram of your image. How do I read the thing and why is it usefull? How can I use it to know what is wrong with the colors in my image and then correct it?
It shows you the pure black(left) and white(right) and the range of values in your image in a graphical form.
You can see all your image values from the lowest value to the highest. You can see if an image is high-key or low-key from histogram even without looking at the image itself.