| It is a ground wire, and it's supposed to be threaded through or wrapped around the string's ball ends to help prevent hum and noise. Many people end up not using the ground wire, since it's not absolutely necessary and can be a bit of a pain.
BTW a friend uses a BP100 on a wonderful Morelli bass as a 2nd pickup. He blends a little bit of the signal from the BP100 with a Gage Realist. He wasn't happy with the Realist alone on this bass, but, with the BP100 added into the mix, he gets a great sound (one of the best I've ever heard).
-bob | 
