The check engine light (MIL or Malfunction Indicator Lamp) is your vehicle computer's call for help. When the engine control module (ECM) detects a sensor reading that goes out of specification or fails an emissions monitor test, it stores a diagnostic code and turns on this light. The light does not tell you exactly what is wrong; it tells you that a diagnostic system has logged a code that must be scanned.
A solid light means you should get the vehicle checked soon, but it is not an emergency. A flashing light is a critical warning. A flashing light means the engine is misfiring badly enough to dump unburned fuel into the exhaust, which will melt the expensive catalytic converter within minutes. If the light flashes, pull over safely and turn off the engine.