How can people reherniate a disc? once the disc has herniated and the "jelly" has come out and been absorbed shouldn't that disc be empty??

The disk isn't empty: The disk has two types of cartilage: a softer inner nucleus pulposus and a fibrous stiffer annulus fibrosis. In a herniated disk, a piece of the disk sticks out of the normal position and may pinch a nerve. This loose piece is annulus and nucleus together: a piece of cartilage like gristle, not jelly. In diskectomy we remove the loose parts, not the entire disk, perhaps 20% of it, as seen on MRI.