2004 Chevy Impala Trouble code 455

Hi guys. My Impala has a error code 455 and I am wondering how to fix this. I know that it cannot be the gas cap because of the fact that it is not loose and fits snugly. I have pretty much narrowed it down to a emission control leak. I do not know much about cars, so what is this "emission control system" and how do I fix this leak? If you could please put your answer in as simple of terms as possible, that would be greatly appreciated. Thanks a bunch!