its a rubber dampener on the end of a rod that is screwed into the hole opposite you stabilizer and when your bow is at rest the rubber dampener is about a sheet of papers thickness/distance from your string. when you shoot the bow the string hits the dampener. what this does is:
POSSIBLY gain a minute amount of speed
Reduces noise and vibration by a huge margin
Produces a cleaner arrow release
and in some cases of people shooting too long of draws it will help prevent arm slappage...