Debugging your code has to do with finding a bug which causes your script to run in an unexpected way (or not run at all), and fix it. The point of debugging should also be to find out WHY a bug exists in the first place because if you can understand why something has happened, you can prevent it in the future.
This tutorial doesn't use JSFiddle, please view the debugging.html page linked to from the Resources section below.
Remember to checkout the Resources section below for associated downloadable content, JSFiddle links, and other resources. Watch the video and follow along!
Also be sure to watch the tutorial on how to use your browser developer tools to learn more!