A flashing check engine indicator light typically means that an engine misfire is happening. This may be a minor misfire, simply affecting performance and emissions, or it could indicate a major misfire. Continuing to drive with a flashing check engine light can do severe damage to a car's catalytic converter.
Most conditions that cause a flashing check engine light are serious. In a major misfire, unburned fuel can pass into the catalytic converter. The converter can ignite this fuel, starting a fire that can increase the temperature inside the engine by hundreds of degrees. This can destroy the converter, crack the exhaust manifold and cause hundreds, if not thousands, of dollars worth of damage to the engine. A car with a flashing check engine light should be shut off as soon as possible and towed to a mechanic who can diagnose the problem.
Steady check engine lights generally mean a more minor condition but one that should still be looked at. One common source of a steady light is an open gas cap. When the cap is not secured properly, the fuel system loses pressure, and the light comes on to indicate the presence of a problem. Turning off the engine and tightening the gas cap should reset the light for the next engine start.