Once you get WordPress up and running, you will have lots of fun. If you are a designer, or if you have your own WordPress site you may play around and experiment. Months later, your theme my “update” meaning that the creators of your theme added some new goodies to it to make it better.
Sometimes if you update the theme, the great things you have done to your WordPress project (or God forbid to your clients WordPress project) can be totally wiped out.
What you do is develop Wordpress Child themes. That way you have your main theme that you downloaded safe and sound and you can mess with the WordPress child themes as much as you want with no worries.