Julia and Winston are bent on avoiding detection of a relationship they both know is illicit (even of there is no law against it) and would get them both arrested. Therefore, they need to encounter each other seemingly by accident, and for short periods. They will meet, for example, shoulder to shoulder in a big crowded rally at Victory Square, and whisper a few things to each other without even looking at each other.
Winston explains that Julia, experienced with clandestine affairs, is adept at "talking in installments," which means picking up a conversation exactly where it left off a week or so ago at exactly the right sentence--even if they had to break off mid-sentence. It is a skill she has had to develop under the pressure of living in a totalitarian state.
Until they get the room above Mr. Charrington's shop, it is difficult for them to have anything approaching a real conversation.