In Ch.6 Orwell describes in great detail how the animals struggled to build the windmill. Large stones had to be hauled up a steep slope and then allowed to roll down to be broken into small pieces. This laborious process, was accomplished mainly by the unrelenting efforts of Boxer:
"To see him toiling up the slope inch by inch, his breath coming fast, the tips of his hoofs clawing at the ground, and his great sides matted with sweat, filled everyone with admiration. Clover warned him sometimes to be careful not to overstrain himself, but Boxer would never listen to her. His two slogans, "I will work harder" and "Napoleon is always right," seemed to him a sufficient answer to all problems."
He represents the common people of Communist Russia who were exploited by Josef Stalin and his henchmen.
In Ch.5,Orwell tells us that,
"On the third Sunday after Snowball's expulsion, the animals were somewhat surprised to hear Napoleon announce that the windmill was to be built after all. He did not give any reason for having changed his mind, but merely warned the animals that this extra task would mean very hard work, it might even be necessary to reduce their rations."
But a little later Squealer explains why Napoleon changed his mind about the windmill:
"The windmill was, in fact, Napoleon's own creation. Why, then, asked somebody, had he spoken so strongly against it? Here Squealer looked very sly. That, he said, was Comrade Napoleon's cunning. He had SEEMED to oppose the windmill, simply as a manoeuvre to get rid of Snowball, who was a dangerous character and a bad influence. Now that Snowball was out of the way, the plan could go forward without his interference. This, said Squealer, was something called tactics. He repeated a number of times, "Tactics, comrades, tactics!" skipping round and whisking his tail with a merry laugh."
"Tactics," of course, means merely that Napoleon stole Snowball's idea of the windmill and then manoeuvred slyly to get rid of Snowball and take all the credit for himself.