Spinal fusion is a surgical procedure done to permanently connect two or more vertebrae. The idea is that joint motion is stopped and (in this example) lower right back pain will also disappear.
With a spinal fusion, bone (or similar) material is place between two spinal vertebrae and metal plates and screws are used to hold them together. The idea is to have the body heal the two vertebrae together as one immobile unit. As you would imagine this changes the way the spine moves and can end up negatively affecting the surrounding vertebrae that are now absorbing an added load.