I have heard my friend talk about being AFK on a Minecraft server, but I have not heard it being used in single player so I assume it refers to online gaming but i don't know what it means and what it stands for.
AFK simply means Away From Keyboard. It's commonly used in chatrooms and online games and it basically means that you won't be available (Not at the keyboard = Not in front of your computer).
Just in case you're wondering what RE or WB means: You'll see these two abbreviations quite often in onlinegames together with AFK. Re which basically means Returned and WB which stands for Welcome Back.