During his speech in Chapter One of Animal Farm, Old Major urges the animals to rise up against their human master (Mr Jones) and take the farm for themselves:
"What then must we do? Why, work night and day, body and soul, for the overthrow of the human race! That is my message to you, comrades: Rebellion!"
This instruction is based on Old Major's belief that humans are responsible for the hard and short lives of animals. According to his observations, humans force the animals to work and give away their produce (like milk and eggs). Humans also prevent the animals from reaching their "natural span" through the cruel practice of slaughter.
Because of this exploitative relationship, Old Major is certain that rebellion is imminent, though he is not sure exactly when the animals will rise up against their human masters. His message, however, is one of hope and inspiration which leads directly to the overthrow of Mr Jones in Chapter Two.