Hot link protection prevents other websites from directly linking to files and pictures on your website. Other sites will only be able to link to file types that you don't specify.
An example of hot linking. Say I like the image on your website, and I want that image on my site. If I use the full URL of your image on my site, then the image is downloading from your site every time someone looks at my site. This means I am using your bandwidth for the image. When you enable Hot Link Protection, then I cannot steal your bandwidth anymore.
To enable / disable hot link protection, please do the following:
You have now protected your images from being hot-linked. Just be sure that all of your additional domains are in the Hot Link list.
Note: Some firewalls (such as Symantec's Norton Internet Security™ and ZoneAlarm™) will block a special referrer variable in order to add more privacy for the user. HotLink protection works with this variable to tell where the request is coming from. As such, HotLink isn't the best solution to avoid bandwidth theft as it will eventually block legitimate requests from visitors using one of those firewalls. Unfortunately this issue is not under our control and there's nothing we can do to prevent it from blocking legitimate visitors. Use it at your own risk.