Symbolically what is revealed in the last chapter of Animal Farm is why Communism doesn't work. An idea, that sounds so good, full of equality and fairness, when it is put in place as illustrated by this book, fails because of the innate corruption that lies in the human/animal soul.
The author uses the pigs to illustrate how, when they take on human characteristics, they easily fall under the spell of corruption and the misuse of power. The selfish greed of humans is already present in the pig, a dirty animal that is most interested in self gratification. It isn't much of a strech for the pigs to resemble humans at the end, because for Orwell, man and pig already share many similarities.
When the animals observe the party in the last chapter of Animal Farm, they learn several truths, none of them pleasant. They learn that the pigs have become essentially human. This also shows them that they (the pigs) have become tyrants, like the human farmers who originally owned the farm before. They see the pigs betraying them, and siding with humans. This shows the watching animals that their revolution has been a complete failure, a bitter truth indeed.
The truth is something they've known inside ever since the issue over the milk and apples yet never wanted to believe or even consider. They should've known it when Boxer left. Napoleon and the other pigs have set themselves apart from the other animals, increasingly distancing themselves, learning to walk upright, wearing cloths and drinking alchohol. They even carry whips to abuse the animals with. The animals see all of their hopes and work totally disregarded, and again they've been put back into the role of the serf, instead of where they've wanted to be. Its all summed up with the words 'All Animals are Equal, but Some Animals are More Equal than Others'. The animals cannot see a difference between the pigs and the humans anymore.