What does broad based posterior disc protrusion effacing anterior thecal sac mean on an MRI report?

Disc protrusion. The disc is in the front part of the spine. The spinal cord and nerves are in the canal within the spine. They are encased in a "sac" containing fluid. If a disc is partially damaged, it will have a weaker outer rim which will bulge out and may compress the cord (esp in neck) or the nerves(in the lower back). Slight bulge may cause disc pain which is usually localized in the back.