This reference occurs in Part II, Chapter 6, and comes in the first conversation that Winston Smith and O'Brien have. The importance of this conversation is shown through the way that Winston Smith believes this is the rebel group against Big Brother making contact with him, and this is cemented when O'Brien deliberately makes a reference to Syme, who has recently disappeared. This reference is a crime, because Syme, as the narrator tells us, is not an "unperson." Note what Winston Smith thinks about this reference to Syme:
O'Brien's remark must obviously have been intended as a signal, a code-word. By sharing a small act of thoughtcrime he had turned the two of them into accomplices.
This is shown to be ironic later on in the novel, because it turns out that O'Brien is not guilty of thoughtcrime: he is a high-ranking member of the Party whose job it is to locate and find members of the Party, such as Winston Smith, who are opposed to Big Brother and then "recondition" them so that that hatred is replaced by love. The irony of this text is based on the way in which Winston interprets this remark as being a symbol of collaboration against Big Brother, whereas the reality is completely the opposite.