Also called MIL light (Malfunction Indicator Lamp). Comes on when a sensor is giving the computer (ECU), information outside of its normal parameters (faulty sensor), or if a misfire is detected (faulty coil etc.).
The car can be driven, if running ok (See Misfire if engine running rough.)But the engine management system should be checked ASAP.
You will need EOBD diagnostic equipment to find cause, however it is worth checking simple things like engine oil level, EGR valve, stop light switches and wiring for chaffing etc. as these can all put the light on, use search by make to see if any known problems.
Don’t drive to far if engine running badly as it may damage the catalytic converter.