Code p0420 "catalyst system efficiency below threshold bank 2" is a code that your cars computer will set when it sees that the catalytic converter is not working efficiently. This can be either because the CAT (Catalytic Converter) is not working properly, but it is more often set because of something besides the CAT.
The first thing to do when trying to diagnose this code is to diagnose any other code that might be present. It is unlikely that this code will be set by itself.
If there are no other codes then the first things to check are your engine oil and coolant levels (if they are low it indicates that they are either leaking externally or being burned in the engine). The next most common thing to check/replace if there are not other engine codes is the Oxygen Sensors. If they are not working properly they can cause this type of code.
If it has been a while since your vehicle has had a tune up then you might want to consider giving it one. Especially if you have noticed it running rough or the check engine light flashing. Engine misfires are sometimes a cause of this problem and by giving your car a tune up you will reduce the likelyhood that your engine will have a misfire.