Please see my response to your other question regarding enotes policy and multiple questions to explain why I have edited your questions and am only responding to one.
At the beginning of this chapter, we are told that "the expected message" had come at long last. As we read on, we discover that O'Brien, whom Winston has suspected as being involved in some kind of rebel movement, has finally made open contact with him, using the pretense of one of Winston's articles as a basis to suggest that Winston comes round to his flat. Note the conclusion that Winston makes after this meeting:
They had been talking to one another for a couple of minutes at the most. There was only one meaning that the episode could possible have. It had been contrived as a way of letting Winston know O'Brien's address. This was necessary, because except for direct enquiry it was never possible to discover where anyone lived...The conspiracy that he had dreamed of did exist, and he had reached the outer edges of it.
This direct contact that occurred between O'Brien and Winston is therefore interpreted as tangible proof of the existence of the conspiracy. Contact has been made, and the "expected message" has been delivered. The stage is set for Winston's further discoveries about the rebel movement.