HD label on our demo website is just an example of a special tag feature introduced in version 1.5. Basically, you just need to choose one of your tags as a special tag and all posts tagged with that tag will display its name in post listings.
Option to define your special tag can be found inside Theme Options -> Misc. Note that the tag you want to use needs to be created first before it can be selected.
You can use it to tag your video posts with HD tag, as in our demo example, or you can use it for any other special thing you may want to display for your post. For example, sponsored, popular, hot, long, short, etc…