Hi GLD and welcome!
Excess charges are when a doctor charges more than the Medicare fee schedule for the procedure. I haven't had that happen but obviously it is something that can happen or it wouldn't be mentioned. Don't quote me on this, but I think that when they do charge in excess of the Medicare fee schedule they are limited to 15% excess charges.
It is a good idea to ask a doctor (or most commonly, their staff) if the doctor accepts "medicare assignment" which is their term for the medicare fee schedule. I have not had one yet that doesn't, but still I always ask.
I think that most doctors that treat Medicare patients pretty much adhere to the Medicare fee schedule.