Winston's dreams reveal several aspects about Winston himself and about pre-Revolution life.
1. Revelations about Winston: Winston's dreams about his childhood portray him as a selfish child who seems to recognize that his mother made sacrifices for him but doesn't necessarily feel any gratitude toward her. This "selfish child" aspect of Winston demonstrates that right before and right after the Revolution, children were already being stripped of their natural affection for their family members. Later, when Winston is with Julia in the room above Mr. Charrington's store, he realizes that his dreams/memories make him unhappy and that he doesn't want to keep going back to them. This is a dangerous thought because if Winston does not even want to think about unpleasant events, the reader wonders how he will respond if the Party catches him and treats him "unpleasantly."
2. Winston's dreams also reveal what might have caused the Revolution and the people's complete obesiance to the Party. When Winston dreams of his days with his mother, times were obviously very diffcult--food is rationed; Winston's father is nowhere to be found, and living conditions are squalid. Orwell most likely creates this pre-Revolution setting to show why the Germans who were suffering through their own Great Depression and the Russians were quite easily swept into "revolutions" which left them with totalitarian leaders who controlled elite henchmen to do their bidding. Like the Germans and Russians, the people of Winston's childhood London most likely thought that anything would be better than their lives at the time.
Winston's "dreams" about O'Brien illustrate how thoroughly the Party monitors its citizens, and in the end, those dreams make it impossible for Winston to distinguish between his dream version of O'Brien and reality.