It refers to a disc that on MRI looks degenerated (usually the fluid in the middle has reduced so the disc looks darker or black and is called a dessicated disc), and typically over time this disc will have reduced height (referred to on the scan as disc narrowing).
This happens to everyone as we age, which is why we get shorter!
The issue is that is disc that shows degeneration on a MRI is not necessarily causing pain. If it is the painful source it is usually the nature of the disc bulging at this level, and not the degeneration as such. Disc dessication (a narrowed, darker degenerative disc) can occur at one isolated level or two (as opposed to the gradual changes seen in the majority of discs with ageing).
What causes this dessicated disc is usually some kind of traumatic injury to the disc, which causes micro-fractures in the vertebral end-plate (the edge of the vertebra that touches the adjacent disc). This end plate fracture is so tiny it would not typically show up on x-ray. Blood vessels come in to repair the end-plate fracture, and these blood vessels contact the adjacent disc (which does not usually get exposed to a blood supply). This contact with the blood vessels changes the metabolism of the disc, and it shrinks (darkening on MRI scans, and reducing in height) over time.
This in itself is NOT thought to cause pain.
The main issue is that over time if the height of a particular disc is narrowed, the space behind the disc for the nerves (the spinal canal) gets reduced. This can eventually give rise to canal stenosis (leg pain that typically occurs in older individuals while walking, from the nerves getting compressed in the spinal canal that has a reduced space. The narrowing of the disc also causes the facet joints (joints that connect one vertebra to another) to be closer approximated, which can contribute to facet joint pain (more common over age 55+).
SO IN SUMMARY
A degenerated disc in itself is not usually the source of pain can increase your incidence of facet joint problems or canal stenosis down the track over many years.
Many people with a degenerated disc that are young do experience pain, but it is usually from the disc bulging (due to certain postures and positions adopted) and not from the degenerative disc as such. eg your disc could cause identical issues without degeneration
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