The Testimony is a practice for the glass people to create more of their kind. They take the people who are near death, duplicate their memories into their Glass avatar, and then put back to their human bodies at the moment of their death with no pain and distress, or any memory of the Testimony process. The dead then can be uploaded into the avatar, and become a Glass person.
Bill Potts was a part of the Testimony, as she herself became a Glass person at some point during Bill’s adventures with Heather. (Twice Upon a Time).