What do Julia, Winston, and O'Brien discuss?

In Part 2, Chapter 8, Winston and Julia go to see O'Brien at his luxury apartment. O'Brien is a member of the Inner Party and so enjoys special privileges unavailable to those outside this charmed circle. When they finally meet Winston confesses to O'Brien that he and Julia are both enemies of the Party. Winston's trust in Julia has given him the confidence to trust O'Brien as well. All three drink wine and make a toast to the past, demonstrating their hatred for the Party and its rule.
O'Brien reveals the existence of a secret Brotherhood, an underground insurrectionary group dedicated to the overthrow of the Party. The Brotherhood is inspired by the ideas of Emmanuel Goldstein, a mysterious oppositional figure who represents the antithesis of Big Brother. Yet, no one knows for sure whether he actually exists. But O'Brien assures Winston that Goldstein is indeed a real person. He promises to send Winston and Julia a book written by Emmanuel Goldstein which will spell out in precise detail how the Party can be destroyed. Then, they will become members of the Brotherhood.
As Winston leaves he asks O'Brien if they will meet again "in the place where there is no darkness," a suitably ambiguous expression which foreshadows much of what is about to happen. O'Brien also knows an old nursery rhyme which indicates to Winston that he's genuinely opposed to the Party, as such historical knowledge is strictly forbidden in the totalitarian state of Oceania.