The windmill represents the massive infrastructure construction projects and modernization initiatives that Soviet leaders instituted immediately after the Russian Revolution, specifically Joseph Stalin’s Five-Year Plans. The way that the animals go hungry in order to build the windmill in the first place mirrors how the Five Year Plans, while intended to create enough food for everyone, were wildly unsuccessful and led to widespread famine in the early 1930s. Later in the novel, the windmill also comes to symbolize the pigs' totalitarian triumph: the other animals work to build the windmill thinking it will benefit everyone, but even after it benefits only the pigs, the animals continue to believe that it benefits all of them.
The Windmill Quotes in Animal Farm
At this there was a terrible baying sound outside, and nine enormous dogs wearing brass-studded collars came bounding into the barn. They dashed straight for Snowball, who only sprang from his place just in time to escape their snapping jaws.