Degenerative disk disease is a wear-and -tear process of the inter-vertebral disks, which are the buffers/connectors of the main bones of the spine, called the vertebral bodies.
When they start degenerating, mostly due to ageing, they lose their inner structure that was conferring them strength and flexibility. Sometimes, acute events can rapidly create injuries that promote accelerated degeneration. Then, they can undergo a gradual process of thinning out or bulging/herniation backward, into the vertebral (or spinal) canal.
Pain can originate from the damaged disks themselves, or from the compression of the nerve roots that are located inside the canal.