It could be any of 13,000 things, from loose gas cap to a broken EGR valve. There is no way to guess. The only way to know is to get an OBD-II scan. Ignore these idiots here who say it might be an "O2 sensor or something" or try a better gasoline- those are universal languages for "I have no idea what I'm talking about". There is absolutely no way to know what component failed. The good news is it's not a serious engine issue unless the light is actually flashing/blinking. Based on your description, we can safely assume that an emissions component has failed, so it's more a concern for the environment than your car right now.