has several meanings. Most often it refers to a person that takes a break from online gambling or chatting (long or short). It could also mean that a person isn't logged in at a particular place (he/she could then be refered to as AFK eventough it's possible he/she is just playing somewhere else - similiar to offline). It is has also start to replace IRL (since technically everything is In Real Life).
X gets a phonecall and therefor write AFK in the chat.