It could mean anything, you need to take it to a garage and have them read the fault code and if it isn't pending, reset the warning light.
Last time mine lit it was a leak caused by a missing cover on the butterfly valve on the air intake manifold, this caused the a "Cylinder 1 misfire" fault code to be written and the engine revs to remain high. All good stuff!!
A common one seen is an oxygen sensor AKA Lambda sensor.