In this video, an enterprising young man places his phone inside an acoustic guitar, points the camera out of the sound hole, and begins to play.

The result is mesmerizing, at first seeming to show the strings oscillating, as if we're seeing the wave structure of the notes. But that's not what's really happening. What we see here is simply the camera's rolling shutter—an effect in which the video camera doesn't capture the whole scene in one instantaneous snapshot, but instead scans (typically) from top to bottom, and because the camera is oriented with its sensor perpendicular to the strings (in "selfie orientation" or at we used to call it, "portrait orientation"), its reproduction of the string movement is all wacky and chopped-up, creating artificial kinks in the strings as the sensor zips along the strings and tries to capture the image.

The rolling shutter creates visual distortion and causes certain kinds of moving objects to "bend" more than they ought to. In any case, this is a trip, but be aware that this is not really how the guitar strings move; it's just how certain kinds of cameras make them look:

Here's a similar video but showing the outside of a guitar (this time a 12-string). Note that, as the subtitles suggest, you do need a lot of light to capture this effect (this forces the camera to use a fast shutter speed, which exaggerates the rolling shutter distortion by taking photos in smaller time-slices):

And here's a video made by Daniel Krnáč for his thesis, showing the difference between a digital camera employing a rolling shutter (CMOS) and one with a global shutter (CCD)—the latter takes an all-at-once snapshot. You'll see that the global shutter looks much closer to what guitar strings really look like:

For more on this effect, check out this detailed Reddit thread pointing to a similar video.