The Brief: AFK is an abbreviation for "away from keyboard."

DEEP DIVE
The term is typically used in relation to gaming or chatrooms. In games like Fortnite, it is easy to “kill” a player who is away from their keyboard, though it may be considered to be poor sportsmanship.
when you go afk to pee and they still stream snipin π pic.twitter.com/8Yf6nVN3nk
— dakotaz (@dakotaz) August 4, 2018