When Old Major explains his ideals of Animalism, he plants in their minds the seed of rebellion against the status quo. Prior to this, the animals were content to live their assigned roles on the farm, submitting to work and slaughter under human rule. However, Old Major's Utopian dreams give them something more to yearn for, and a goal to work towards.
"That is my message to you, comrades: Rebellion! I do not know when that Rebellion will come, it might be in a week or in a hundred years, but I know, as surely as I see this straw beneath my feet, that sooner or later justice will be done."
(Orwell, Animal Farm, msxnet.org)
The slow foment of their rebellious ideas comes to a head when Farmer Jones forgets to feed them; in their minds, this is the ultimate example of the human indifference to animal suffering. By framing their situation as "us vs. them," giving the animals an enemy to overthrow, he changes their mental situation from work to slavery. Without Old Major's ideas to spur inventive thought and action in the animals, they would never have considered rebelling.