What does the "check engine" light mean?

I have a 2003 Hyundai Elantra. It's been a fantastic car, and I've had virtually no problems with it. I have done the standard maintenance on it faithfully, but seeing as it's a 6 year old car, things are bound to start happening. Last night on my way home from work, the "check engine" light... show more I have a 2003 Hyundai Elantra. It's been a fantastic car, and I've had virtually no problems with it. I have done the standard maintenance on it faithfully, but seeing as it's a 6 year old car, things are bound to start happening. Last night on my way home from work, the "check engine" light came on. I have never encountered this on a car before, so I'm wondering if anyone out there can help me with what this could mean. Should I panic? Is this serious (I'm not driving it today, by the way). I would look it up in my manual, but it seems to have gone missing (could be in the trunk; I'll have to take a second look). Thanks for your help!
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