Boxing an AI is the idea that you can avoid the problems where an AI destroys the world by not giving it access to the world. For instance, you might give the AI access to the real world only through a chat terminal with a person, called the gatekeeper. This is should, theoretically prevent the AI from doing destructive stuff.
Eliezer has pointed out a problem with boxing AI: the AI might convince its gatekeeper to let it out. In order to prove this, he escaped from a simulated version of an AI box. Twice. That is somewhat unfortunate, because it means testing AI is a bit trickier.
However, I got an idea: why tell the AI it's in a box? Why not hook it up to a sufficiently advanced game, set up the correct reward channels and see what happens? Once you get the basics working, you can add more instances of the AI and see if they cooperate. This lets us adjust their morality until the AIs act sensibly. Then the AIs can't escape from the box because they don't know it's there.
It may simply deduce that it is likely to be in a box, in the same way that Nick Bostrom deduced we are likely to be in a simulation. Along these lines, it's amusing to think that we might be the AI in the box, and some lesser intelligence is testing to see if we're friendly