Barney-style definition of trouble code

Hello all...
I am looking for someone to help me with a better definition...and maybe even a solution to a trouble code I am getting on my 1998 Pontiac Trans Sport.
The code reads: P0420
The definition in my Haynes manual states: Catalyst system efficiency below threshold.
Ok...So something must not be working right...and I am hoping that it isn't a bad catalytic converter.
Any suggestions as to what may cause this code, and what I can do to fix it?
Thank you in advance!

http://www.obd-codes.com/p0420 more help there

Hey...that is an awesome site! Thank you for posting it. I now know where to at least start so I can fix this problem.