The windmill is one of Snowball's great ideas, a way to take some of the workload off the animals, allowing them more leisure time and so more time to enjoy their lives. His idea is entirely in keeping with Old Major's original philosophies, that the animals, working together as a team, will benefit everyone and be more free because of their shared labor. However, Napoleon sees the windmill in a different light, and when it is finally finished, it does not help to relieve their work:
The windmill, however, had not after all been used for generating electrical power. It was used for milling corn, and brought in a handsome money profit. The animals were hard at work building yet another windmill...
(Orwell, Animal Farm, msxnet.org)
Instead of using the shared labor of the animals to create electricity, taking the burden of work off the animals, Napoleon uses it to make money for his own use. Since he and the pigs are living as dictators, keeping and consuming far more than their fare share, the windmill no longer represents shared labor, but slave labor.
The eventual meaning behind the windmill is another perfect proof of Napoleon as dictator. Napoleon is always pulling the strings behind the scenes and orchestrating all things to his own benefit and NOT to the benefit of the other animals (as he makes them believe). The windmill is no exception.
Of course, the windmill is not Napoleon's original idea, it was Snowball's. However, it isn't long before Napoleon sees the benefit of the windmill for his own use. Snowball proposes the windmill to ease the animals' heavy burden of work. The original hope was for the windmill to produce "electrical power." Snowball, of course, hopes that, as a result, the animals will have more time for fun and enjoyment. The unfortunate truth is revealed here however, for the windmill:
had not after all been used for generating electrical power. It was used for milling corn, and brought in a handsome money profit.
So much for the bereft ideas of Old Major: that shared work would benefit the masses, ... instead, this shared work simply makes more money for Napoleon. The animals are slaves to Napoleon and his cronies. Not only is the windmill being used to grind more and more corn, but Napoleon demands a second windmill to be built, again for the dictator's own use.