On the last page of your remittance advice all of the denial code definitions are listed in fairly easy to understand language.
The CO means "contractual obligation" - that would mean what the doctor or facility has agreed to in order to be a Medicare provider.
Then the number after the CO will be defined.
I've never gotten these codes, so I can't tell you what they are. But you should be able to get it from the bottom of your remittance advice.
Hope that points you in the right direction!