These are the little rubber buttons right?

If so, the purpose is to sit between the release and nock and keep the nock from being pinched off the arrow by the release.

There are two kinds, solid ones which you need to remove one end of the string and thread them on, and ones with a slit in them that you can put onto the string, then put a metal band around and crimp that shut.

They also can be used as speed buttons and vibration dampeners in the string.

--Bob