What is Major's warning to the animals when he gives his speech?
Old Major's speech acts as a prophecy--and like most prophecies can only be judged as such once the events have come to pass. In this case, Old Major warned the animals about all the best and worst things which were to come.
The best: a world without men (those who were their oppressors), a world in which all animals would get along and in which all animals would be equal. They almost had this for a very short time.
The worst: a world where the vices, habits, and trappings of "men" (such as walking on two legs, wearing clothes, drinking alcohol, sleeping in beds, and others as outlined in the Commandments) would bring destruction. This became their reality all too quickly after Old Major died and Jones was ousted from the farm.
In short, Old Major was both predicting the potentially utopian future of Animal Farm and sounding a warning for those who could understand it. Unfortunately, for the majority of the animals on the farm, the worst came to pass.
Old Major first warns the other animals that he will not be around much longer and has chosen this time to "pass on to you such wisdom as I have acquired." He tells them that all of England's animals are slaves to their human masters who steal their labor and by-products from them. Things will never change unless they stage "Rebellion!" He warns them that anything that "goes upon two legs is an enemy" of the animal. "Whatever goes upon four legs, or who has wings, is a friend... All animals are equal." Man, and all of their "habits," is evil. He tells them of a dream he has had--a dream about the Earth when "man has vanished." His words are such that they inspire the animals of Manor Farm to take immediate action (following Old Major's death) against the humans who have been abusing them.