majority of CELS are related to Emissions problems and usually aren't very serious - but don't neglect. if the car is running ok, then obviously it's something that may need to be looked at eventually, if symptoms occur.
I have had Phantom CELs in the past that don't leave any codes to be read. To reset the CEL you can disconnect your battery for a time and then reconnect. Make sure you have your radio code before doing this