AFK is an abbreviation of the phrase "Away From Keyboard". This means that the player is away from the keyboard doing something else and implies that they are currently unable to play a computer game or interact with their computer.
In Warhammer Online one may mark themselves AFK or returned for all to see by typing /AFK into the chat console. Remaining idle in game for long enough will also automatically flag you this way. In the latter case, the flag is removed as soon as the player resumes moving about and interacting with the game.