Fag is a derogatory term for a gay person. Or a term used for insults in general. And rats are commonly associated with dirty places. So when calling someone a "Fag rat" you are calling them a dirty/ gross "fag". Also used as an insult in general nkt specifically a dirty person
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.