Abbreviation of "Get your ass over here". Polar opposite of GTFO, an invitation with urgency rather than dismissal.
More commonly used in communication where people know each other, rather than anonymous. Can be used in an online game, for example sending some a /tell to get to a particular zone, or to imply physical movement - to get their ass to someone's house/flat/apartment/room/dorm
PersonA: "I have a severe hunger for a new game, but am stone broke"
PersonB: "I have found a $50 bill in my pocket"
Ayf = All your Face, as in, All your face are belong to us, the battlecry of the alien facehggers from the alien films. a la all your base
Rob: I'm playing the AVP game
Will: Wow. What as?
Will: Got facehuggers?
Rob: Yes.... why?
Will: So you can say: "AYF"
Will: As in, "All your face"
Will: as in, "All your face are belong to us"