its a warning light (orange in colour) to say that there is a problem with the engine.... it can go off for any one of a load of different reasons. for example when my dads petrol corolla got some diesel in it by mistake, the light stayed on for ages afterwards. in my corolla, it came on and i discovered that there was a problem with the timing chain..... your problem could be serious or trivial, but you won't know exactly what it is until you take it to the garage to be hooked up to the computer....
I recommend you take it sooner rather than later to be on the safe side.....
Good luck with it!