It's called the center serving and needs to be replaced asap either by reserving or another string. It protects the string itself. Most compound xbows need a bow press to remove/replace strings. You may be able to reserve yours w/o that either by taking the riser off bow or using a gizmo that's hard to describe. I'm not at my own computer, so here's a post I made when I was w/ a pic of it. Post #17
http://www.crossbownation.com/community/showthread.php?t=1472&page=2
Good luck!