Herniated or slipped discs don't actually slip, rather they split or rupture causing the inner gel-like substance (nucleus pulposus) to leak out. The elasticity and water content of the nucleus pulposus decreases with age, leading first to disc degeneration then prolapse as a slight bulge or protrusion begins to form, which might begin to crowd the spinal cord. Later stages are the extrusion of nucleus pulposus through the tyre-like wall of the annulus fibrosus, and then actually breaking through this wall into the spinal canal.
Not every herniated disc causes symptoms. In fact, many people discover they have a herniated disc after having an x-ray for an unrelated reason. If symptoms occur, they generally appear at the level of the damaged vertebrae, and should be treated if pain is increasing or if pain becomes disabling or leads to leg weakness, numbness or tingling and loss of bowel or bladder control.