Check engine light on...
When the check engine light comes on it indicates the computer is reading something out of tolerance affecting the emissions. It can be something as minor as a loose gas cap and that should be checked first. It the car is running normally it is not an emergency to bring the car to the shop, but I am not saying to ignore it, it still needs to be addressed. We can check the system while you wait if you call ahead and let us know.
If the car is not running normally it should be brought into the shop as soon as possible to be checked. It could be something that could damage the engine or exhaust.