When your check engine light is just on it usually isn't a bad thing and can just mean that your gas cap is loose or missing or broke.
Or you need to top off your oil or transmission fluid.
Now if the check engine light is flashing then it usually indicates a more serious engine problem.
When a check engine light is flashing it's warning you to get the engine checked out soon because it's most likely caused by a severe engine misfire.
When your engine is severely misfiring it allows the unburned fuel to be dumped into your exhaust system and when that happens your catalytic converter rises to a dangerous temperature which can damage the catalytic converter as well as damage your engine which can require a complete new engine or other expensive repair.
If your check engine light flashes get the engine checked as soon as possible.