I was reading about time lapse photography and there was a reference to the histogram.  I had heard the term before and knew it related to a graph of some type but really didn’t know what it was.  So, I started to do some research.

Basically, it is a type of bar graph that displays exposure of the pixels in your image, i.e the brightness of an image.  Dark tones are displayed on the left and moves to light tones on the left.  Typically, a good exposure would have more data in the middle then the edges but this might not apply to sunset images.  An underexposed image would have a spike on the left side (dark side).  An overexposed image would have a spike on the right side (light side).  A well-exposed image will reach from both sides.  Spikes can also represent clipping of detail in the images

But, this info might not apply to a sunset since it will have spikes and be slanted to the left.