When you enable these lab feature, Gmail will automatically detect Youtube, Flickr, Picasa, and Yelp links to display nice previews right inside of Mailplane.
From the official Gmail blog:
The truth is, I'm pretty lazy. That's why I like YouTube previews in Gmail chat -- why open a new tab when I can watch the video my friend sent me right there? Gmail currently automatically detects package tracking information, addresses, and event information and shows quick links to delivery status, maps and directions, and Google Calendar. So why couldn't Gmail automatically detect links in emails and show videos, photos, and ratings right inside these messages as well? We built some new Gmail Labs features that do just that. For our first set, we picked stuff that often shows up in email: YouTube videos, Picasa and Flickr links, and Yelp reviews. Turn on Picasa previews from the Labs tab under Settings, and rather than having to click on a Picasa album link to see the photos it contains, you can see photos right in the message itself.