The soft gel-like material, the nucleus pulposus, bulges out in a hernia.

http://en.wikipedia.org/wiki/Human_vertebral_column

Spinal disc herniation, more commonly called a "slipped disc", is the result of a tear in the outer ring (anulus fibrosus) of the intervertebral disc, which lets some of the soft gel-like material, the nucleus pulposus, bulge out in a hernia.