In normal anatomy, the disc acts as a shock absorber and bears most of the body weight.  When the disc takes too much stress, some of the outer fibers or belts of the disc break, much like the belts of a steel belted tire fracture when one hits a pot hole.  When the belts fracture, there is a bulge.  In the case of a disc, the disc bulges back into the neural canal or opening where the nerve root sits.  The pressure on the nerve root creates pain.