From Ernie Ball:

The Ernie Ball FretWrap by Gruv Gear is a professional string dampener/muting accessory that every studio, session player, and tapping aficionado should have! Slide on the FretWrap to effectively cut overtones and sympathetic resonance during recording, live performance, two-hand tapping, or any creative situation where you need that extra muting hand for cleaner takes without the unwanted string noise or ringing. The adjustable strap lets you fine-tune the pressure and dampening of the strings. You can quickly slide the dampener over the headstock when not in use. No special guitar modification, installation, or tools required - Yet amazingly effective!

SM - Fits 4-string basses, 6-string electric and acoustic guitars, and ukuleles