the downstream O2 sensor (one after the catalytic converter) is being used to monitor the efficiency of the cat. The computer reads the signal from the upstream O2 sensor and comparwes it to the signal from the downstream sensor to see how much the cat is able to clean the exhaust gases.
Your code means the downstream signal is not within acceptable limits. There are only two reasons.... either the O2 sensor is doing it's job and the cat is really getting weak, or the O2 sensor itself is getting old and not working properly.
The logical, economical approach would be to replace the downstream O2 sensor first and see if that gets rid of the problem. Clear the code and drive, and see if it comes back or not. It may take a week to return.
If the code returns, then your cat really is getting weak and should be replaced. When replacing the cat, you would normally replace the O2 senror too, but now you already have a new one in there.