Away from keyboard

From Guild Wars Wiki

AFK is an abbreviation for away from keyboard. A player who is AFK is thus unable to respond to messages or in-game situations. It is considered courteous to inform party members when going AFK.

The term AFK-ing is often used as a synonym for leeching.

Also, typing /afk causes your character to use the same emote as /sit but does not actually set your status to "away." To set your status to away you must use the drop-down list in your Friends menu.