There are pages of explanations about what you're seeing here, but first just watch this. Obviously, it's a camera looking at the strings of a guitar from inside of the sound hole.
The illusion is caused by the way the iPhone captures video; it's called the rolling shutter effect, and in simple terms, each frame is captured at slightly different times. Each line of the video is captured ever-so-slightly later than the one before it. At least that's how I understand it. There's a good discussion of this here. I've seen videos of spinning propellers and bike wheels that are equally as beautiful and mystifying.