What does P0300 mean for a Chevrolet Cavalier diagnostic test? All it says is a misfire. I need to know.?

My husband changed the plugs in it because it was running rough. After that, it almost didn't start one night on my way home. I finally got it home and he started it the next day and let it run for a couple of minutes when he smelled something hot. He shut it off and looked under the car. The pipe coming out of the catalitic converter was so hot and red you could almost see through it. I really need help on this. Any suggestions?