What does this mean? "residual/recurrent left central/subarticular l5/s1 disc protrusion?"

Suggests you may: have had surgery for a left L5-S1 herniated disc, and your new MRI still shows disc protrusion in the area close to your prior herniated disc -- hence the wording of residual (suggesting not all disc material was successfully removed), or recurrent (suggesting all disc material was successfully removed, but new disc fragment is now protruding in the same area -- different from the original one.)