It means something is wrong with the engine, usually in the pollution control system, but doesnt mean stop driving right this minute like a red oil pressure too low light.
I my state you cant pass the emissions test and license stickers if this light is on. Nor can you transfer title, except to salvage.
When my 2004 civic had the light at 30K, the dealer said drive it for three or four "cycles" in case it was a temporary problem like a loose gas cap. They said they'd charge an hour labor just to track it down. But since it was still under warranty they did the diagnosies and injector sensor replacement free.
A 1990 civic had had an oxygen sensor failure at 200K and the replacing cost $300. (My threshhold for dumping the car was more than $1000 or work per year.)
People advide trying to get a cheap code readout from a parts shop to save the diagnosis cost, especially if its something simple like a leaky gas cap. The parts shop may not know what all the codes mean. Someone told me a 2004 civic has 285 codes now.