To answer this question, take a look at chapter 6. Although the farm has a large supply of limestone, the animals initially find it impossible to break the up the stone into smaller, more manageable sizes because they do they cannot stand on their hind legs to use a pick or crowbar.
After a few weeks, however, one of the animals realizes that this task can be achieved if they use the power of gravity. To do this, the animals dragged a boulder to the top of the quarry and let it fall off so that it would smash on impact into lots of smaller pieces.
Although this solution seems simple, the animal struggled to drag the large stones to the top of the quarry. In fact, it could take all day to drag a single one. As the narrator says, it was a "slow and laborious process."
Despite the difficulties involved, the animals did not give up. In this scene, Boxer shows himself to be the most determined and hardest worker of all, refusing to stop even when he was in danger of overstraining himself.