1997 Firebird Computer code clarification

I have a 1997 Firebird. 3.8 Liter V6 Engine. Service Engine light came on and it shows a code number that according to the service manual says
"Catalyst system effeiciency below threshold". What does this mean and how do you fix it? I am thinking it is the catalytic convertor, but not sure. Any help would be appreciated.

thanks in advance,

JMAC

Does it seem to loose power after driving a while? Put oxygen sensors in it yet?

Actually it never loses power that I can really tell. I did put new oxygen senors in it. But it didn't correct the problem. I am looking at other things like maybe catylitic converter. I will post as I find something.

thanks,

JMAC