Angela it means he just charges what Medicare cover ie he sends the bill to Medicare for the amount they cover so you don't have a gap to pay for your specialist. My step dad also asked his surgeon to do this for his prostate surgery 2 months ago cos it wasn't covered by his top level PHI (not sure why but anyway) as it was going to cost him about $2k out of pocket and being on a disability pension he just simply couldn't afford that much.
Good luck with it all. When is your op?