FXX is a cable channel that launched September 2, 2013. It is a sister channel to the FX network, catering to a younger audience, and focusing its programming on comedy series. It replaced the Fox Soccer Channel.
It's Always Sunny in Philadelphia moved to FXX starting with its 9th season.
If you are not sure if FXX is available on your cable or satellite provider, or you need to know what channel it is, go to this site: http://www.getfxx.com.