What is the significance of the countryside setting where Winston and Julia meet for the first time?
Throughout the novel, Winston has a recurring dream of standing in an open pasture in the country, away from the dirty, smug city streets of Oceania. He calls this fantasy setting the Golden Country and has an overwhelming sense of joy in being close to nature. In the Golden Country, there is a quiet stream, scattered molehills, and swaying elm trees. Winston also dreams about a girl with dark hair who throws her Party uniform off before embracing him. Later on in the story, Winston and Julia travel to a remote pasture in the country, which resembles the landscape of Winston's dreams. It is in this pastoral location that Winston and Julia begin their affair. Their consummation is significant because it is a strike against the Party, and Winston views it as a political act. The Golden Country is also Winston's interpretation of how the world should look. Winston intrinsically believes in independence, privacy, and the opportunity to enjoy the natural environment. When Winston meets with Julia in the Golden Country, it is the epitome of his ideal life and solidifies his stance against the Party.
I think that the main significance of the setting where they meet for the first time is that it corresponds perfectly to the setting that Winston has a recurrent dream about. He tends to dream about this place that he thinks of as the "Golden Country." (We see this in Part 1, Chapter 3.)
In this dream, he sees a young woman take off her clothes and beckon to him. He is taken by how free and graceful she looks and sort of envies her this freedom. When Winston meets Julia there, he is essentially having this dream come true. With Julia in the Golden Country, he will try to become free.