What does the windmill in Animal Farm represent?

The windmill is a symbol for Joseph Stalin's Five-Year Plans for the National Economy of the Soviet Union. Just as the windmill was promised to make the animals' life easier, the Five-Year Plans were intended to improve Soviet industry to the point where workers had shortened work-weeks.

Analogous to the destruction of the windmill, several of Stalin's Five-Year Plans failed to improve work production. Also, as the animals decided to re-build the windmill, Stalin also kept producing many more Five-Year Plans, some of which were successful.