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.
At the beginning of the story, Old Major explains to the animals that humans make their lives miserable and meaningless. Old Major encourages the animals to free themselves from human tyranny by rebelling against Mr. Jones and his men. He then proceeds to give the animals the commandments of what will be called Animalism and teaches them a song, "Beasts of England." The animals respond in excitement and repeatedly sing "Beast of England." After Old Major dies three days later, the intelligent animals on the farm begin planning for the impending Rebellion. Orwell writes that the next three months were full of "secret activity." Old Major's speech inspired the animals to take control of their own destiny and rid themselves of human tyranny. Napoleon, Snowball, and Squealer then develop Old Major's teachings into a system named Animalism and secretly discuss the tenets with the other animals at night. Old Major's speech gave the animals a new perspective on life which encouraged them to rebel against Mr. Jones and his men.
The effect of old Major's speech convinced the animals to rebel against Mr. Jones and the other humans. Also, his speech gave the animals hope and courage which helped the animals to overthrow the humans from Manor Farm.