The significance of the animal rebellion in the novel is that it transforms the power structure. In chapter 1, Old Major's speech makes clear that the humans control the animals. The conditions of life on the farm are ones in which the animals are exploited, unable to keep the fruits of their labor. In addition to this, they are butchered at the behest of the humans. The conditions in which they live are the rationale for Old Major's battle cry to revolution. This cry is something not fully heard until Chapter 2 in which the animals begin the rebellion because of the humans forgetting to feed them.
The significance of the rebellion is that it causes a change in who has power on the farm. To borrow language from the Marxist, it is significant because it represents a change in the ownership of the means of production. For the animals, their rebellion is a starting point for them to own the farm, to control labor, and to, theoretically, own the fruits of their own labor. In addition to this, it is significant because the rebellion affords the opportunity for the pigs to lead the animals in their control of the farm. It will turn out to be a fateful decision, helping to make the rebellion even more significant.