The back light adds highlights that help define objects. Without a key and fill light, you can see this more clearly:
The back light is about 180 degrees from the camera, as seen on the left side of the following screenshot. With only the back light, you see highlights on the top of the head and the shoulders, as seen on the left side of the following screenshot. Like the key light, the back light is high. Its job is to create highlights that define the subject with a rim of light.