It is an artifact of the "shutter" and how the image frame is not captured across the whole chip in a single instant, but rather scanned.
Another video of this artifact has a link to an online discussion http://youtu.be/TKF6nFzpHBU
One clue that it doesn't represent really is that you only see waves on a string whose position is otherwise static. There is no oscillation of the first natural frequency.
Think of a rope tied between two points. If you sway it back and forth, that's the first natural frequency, and is the primary frequency component of the strings tone. Then hold one end and wiggle it back and forth faster, and you would be able to get more waves or ripples in the rope/line. Those are higher natural frequencies.
So for a single guitar string, you expect to see the entire string moving back and forth like the swaying rope. Plus on top of that movement, you have higher frequencies added as well. That video showed what looked like complex ripples moving on the strings in their original position, like lines on a page.
The newer iPhones have a slow motion camera mode. I'll have to go see if that will show a clearer view of the bass string. (That is the largest to see, and moves the slowest)
Update: here's a video http://youtu.be/TmA64WfVBq0
