It is part of your vehicle's diagnostic system. Basically, it's a way that your vehicle communicates to you that it needs attention. What kind of attention? When your engine warning light is illuminated, it could mean a number of different things.

One thing you want to immediately check is your gas cap. Your fuel system is pressurized. A loose gas cap can create a signal that pressure is low. It could also indicate a number of issues in this system. For instance, your oxygen sensor may have gone bad.

Bad spark plugs diminish engine performance. You could experience reduced power if they are not sparking properly. Your engine warning light could indicate a need to replace spark plugs.

Just as you'd think, this sensor measures the flow of air into your engine. This in turns tells your engine how much fuel is needed. A faulty mass airflow sensor can illuminate your engine warning light.