Hashtags are in vogue these days. They are already basically in all the major social networking services (Facebook, Instagram, Twitter and even Google+), and now they are also making a much more prominent appearance on YouTube. Both on the web and on mobile devices, hashtags begin to appear on the titles for the videos that have them in their descriptions.
Basically, for videos that have tags in their descriptions, YouTube will show the first three directly on the title. Clicking on each tag will take you to a YouTube search, revealing other videos with the same hashtag. If there are hashtags in the title, they can be clicked and hashtags will not be shown above the title.
Note that only the first three hashtags appear above the title.
I could have sworn that I've seen this before, but whether I've been imagining things or not, now it's live. Google has uploaded some help pages if you want more information on this specific topic, although there really is not much more than that.