AFK, which stands for Away From Keyboard, is a term that standards for someone who is not currently actively participating in the game despite being online. This can be because they aren't on the computer, or are distracted by something else such as browsing the Internet, playing other games, watching videos, etc.
The game considers someone AFK if they haven't moved in 30 seconds since the start of the round (or died before moving at start of round). If you move when the round starts but then go AFK, the game doesn't consider you AFK in regards to any AFK issues mentioned below.
Players who are AFK for a long time are disconnected from the game, with the message "You've been AFK for too long :'(".