Shorthand for away from keyboard, AFK lets others know in chat and games that they are away from the keyboard (computer) and will not respond to any talking or action. Below is an example of how this would be used.
User1: That's pretty funny.
User2: Heh yeah, AFK biobreak.
User2: Ok I'm back.
User1: Welcome back.
Some people prefer the term AFK instead of IRL (in real-life) because they consider the Internet as their "real" life.