At the beginning of the pigs administration, one of their seven commandments stated that all animals were equal. As the book continues and even after attempts at education, the pigs conclude they are indeed the most intellegent of the animals. Thus, the commandment changes to all animals are equal, but some are more equal than others. At this point, the pigs start taking the milk for themselves, and eventually start living like the humans in the farmhouse... they even get to the point that they are brewing and drinking beer.
By the end, we see that what the pigs set out to do in leading a rebellion of the existing government at the beginning resulted in being recreated by the end. The pigs were mad about their inequality with humans, but by the end, had issued the same inequality to all the other animals.
At the beginning of the book, the word "equal" really means what we think it should mean -- that everybody will truly be treated the same.
By the time the story ends, however, "equal" means something completely different. As the saying goes, by that time we have found out that some animals are "more equal" than others. This is clearly not the same meaning as the word had at the beginning.
Instead, "equal" now seems to just be a kind of propaganda term -- a word whose real meaning has been taken away and replaced with another. It is just a word that is meant to make the animals feel some way (like they really are equal). This, to me, is sort of like how people in the Soviet Union called each other "comrade" to show they were all equal when they really weren't.
The most obvious example of the change is how the pigs get everything. This leads to the change in the slogan from all animals being equal to some animals being more equal. Both of these are examples of the change.