It essentially means idling/not doing anything and expecting a reward for no effort...... well that's what it means in this particular instance. AFK just means Away From Keyboard as Phil Said.... or controller if your on console.
That's funny, because last night a guy laid back and let me fight the fallen walker and all of his minions by myself, while in the devils lair. He then came strolling out as soon as I killed the walker. I was going for the "do a strike without dying" bounty. I was close to death a couple times, but I did it. He did help me at the end with the servitor.
Yeah, it's only a legit strategy if your fireteam knows what you're doing. And you can play it safe, and still get a few shots on the target.