The original commandment reads, "All animals are equal." It is the seventh of the Seven Commandments the animals devise when they take over the farm and initially a bedrock belief. By the end of novel, the commandment has been changed by the pigs and now reads, "All animals are equal but some are more equal than others." The word "more" makes nonsense of the original statement. If two things are equal, logically one cannot be "more" than the other. For example, if two items are said to weigh the same amount, one cannot also, at the same time, weigh more. It's a non sequitur: in other words, the second proposition cannot logically follow the first.
Orwell's point is that if we are not careful, language can be manipulated to mean the opposite of what was intended. The new language can then be used by people in power to oppress others and undermine democratic principles. The pigs use the new commandment as an ideological justification for the privileges they alone have seized. Animal Farm is a cautionary tale meant to urge people to listen very carefully to what people in power are saying rather than accept it blindly.