What does it mean when check engine light comes on?

it means an electrical wire has alerted the engine that something is either broken, or not attached etc. Go to an Advanced auto parts and they will check it for free. it takes 5 minutes. Has happened to me once or twice. I have an '02 Liberty Sport. My wife did not tighten the gas cap all the way. Tightened it up and it went away once the tank pressurized. Not saying that is the only reason, but IS a problem and a solution. The check engine light will illuminate for the reasons stated in the first paragraph. So if the gas cap is tight, probably something else. If it is the gas cap, or you suspect it is, it might take a minute for it to go off.