Well, I don't actually play the guitar, but they have to have their strings tightened so that songs can be played without sounding out of tune. I'm guessing that the interpretation was that his strings were too tight, and it's been macabre-ified to cut into his skin!
This series bizarrely fascinating, distressing, and profoundly disturbing in a deep sort of way... I keep trying to look away but my eyes are inexorably drawn back to the page. That is a characteristic of great art.