The Heat Map shows the "hot" and "cold" areas of your email. Hot areas had the most activity and Cold had the opposite. You will never get the same number of clicks on each of your links. This difference may be due to your links and the interest that your readers on them, or even the arrangement of your newsletter: some areas being more visible than others.
The Heat Map is very useful for learning what your contacts like and how to improve your marketing efforts. For example, you can work on areas of your email that have not been successful, and use links in areas with more activity, etc.