The bass bar, made of spruce, is the bracing glued to the underside of the top plate to prevent the arching from collapsing. It runs underneath the foot of the bridge on the bass side and helps take the pressure that the strings place on the plate through the bridge. When this is glued and shaped, we can go ahead and fix the top plate to the ribs. In the video you can see how Peter and Mark do this.
The parts of the violin are glued together using a traditional animal glue. Animal glue is ideal for this because it is reversible; using synthetic glues would mean that the body could not easily be opened up later for repair.