2002 Saturn Vue Smog test

My car failed the smog test and the data trouble codes showed P0300 "random/multiple cylinder misfire detected" and P0420 "catalyst system efficiency below threshold (bank 1)". The check light engine was off when I first started the car that morning, drove around, turned the car off at the smog station, and when the tech turned the engine on, the check light engine came on. What do these codes mean and how can I fix the problem without replacing the whole catalytic converter?