As is the case with every aspect of Animal Farm, Old Major's speech can be analyzed on different levels.
Taken literally, Old Major is lecturing the rest of the animals about the basic differences between humans and animals. He is encouraging the animals to develop and carry out plans to rid themselves of the oppressive presence of humans on the farm, pointing out the inequalities in terms of labor and consumption between the working animals and the powerful humans.
Man is the only creature that consumes without producing. He does not give milk, he does not lay eggs, he is too weak to pull the plough, he cannot run fast enough to catch rabbits. Yet he is lord of all the animals. He sets them to work, he gives back to them the bare minimum that will prevent them from starving, and the rest he keeps for himself.
Allegorically, Orwell uses Old Major's speech to set the stage for the need of a true revolution to benefit the working class, as opposed to the disappointing outcomes brought about by the Russian Revolution of 1917. Old Major represents Karl Marx, the developer and advocate of the political/economic philosophy known as communism in the real world and as animalism in Animal Farm. The animals were the working classes being exploited and oppressed by Tsar Nicholas II (represented by Mr. Jones). Even during Old Major's speech, however, the pigs were setting themselves apart from the other animals by positioning themselves in the prime audience location. As Old Major was encouraging the animals to unite against the humans, separations between different groups of animals were already coming into play.