Actually, it's a memento of a pretty awesome day for one Redditor's room mate. The girl came to get her braces off, and ended up getting a photo op with Jack Black.
Reddit user "megloface" posted the photo to the social news site, where it quickly racked up karma. Some commenters derided her room mate for playing paparazzi with Jack Black's dentist visit, leading "megloface" to explain the context of the picture:
"[Her] dentist is an old family friend, and they're all pretty chummy with Jack, and call him 'Blackjack.' The assistant called her to the room to meet him casually, and when the assistant said something about not being able to take a picture because he was on the chair, he OFFERED to scoot over like this so she could take a picture. She in no way forced it on to him or 'ruined' his day."
TL;DR: Jack Black is awesome.