I'm not an expert, but I think a step-off is where there is a significant deviation between surface planes of the ajoining bone segments at the osteotomy site. Looks a bit like a step, like if you cut a plank of wood in half and then shunted the two pieces together without lining them up properly. I think significant deviations are more likely to lead to deformities after surgery because the bone won't completely smooth them out as it heals. I don't see why you should have a step-off on one side, unless your jaw was wonky before the surgery. Is your jaw straight now?
Maybe the metal left in your body was a bracket of some kind. Either that or it was the head for some kind of surgical tool, like a saw or a drill etc.