In the context of Animal Farm, this quote helps to explain the way that Napoleon slips into greater and greater degrees of corruption, utilizing deception and violence in ways that are in no way supported by the ideals of Animalism.
The more power he gains, the more he is willing to abuse the animals. The more power he gains, the less he feels beholden to the will and the safety of his "people".
This is really a truthful commentary on the inherent sinfulness of man. Everyone has something in him or her which can lead to corruption (i.e., greed, selfishness, hate, envy). Those who want power want more of it. This is true in life, as we've seen throughout history, and it's true for Napoleon and the pigs in Animal Farm. Their rise to power was accomplished so easily, it's no wonder they kept seeking more. If the animals had ever put up more than a token resistance, perhaps the power would have been at least a bit more evenly distributed. As it is, the saying is absolutely true for Animal Farm.
This quote from Lord Acton is correctly interpreted by other editors above - it represents the corrupting nature of power but places more emphasis on the danger of power without any form of check whatsoever, as represented by the pigs but more particularly by Napoleon. It unfortunately paints a very realistic portrayal of our innate corruptibility if we are given power to do what we want.
I agree with Post #4. The balance of power in the US is in three different branches--Executive, Legislative, and Judicial. While some Presidents seem to be overly controlling...trying to get control of everything from healthcare to education to every other aspect of our life in this country...they are not in complete control since they answer to the other branches and to the American people.
In Animal Farm, however, the pigs put themselve into government without a balance of power. They are the end-all and be-all. They simply change the laws in order to justify their breaking of the laws before the changes. They are absolutely corrupt. No one can stand up to them, and they become what they hate the most...two-legged men.
The previous posts obviously interpret the quote correctly, but there should be more emphasis on the word "absolute." It's not simply that a leader gets more power; rather it is that he obtains complete control over a country or group of people. Stalin (whom Napoleon represents) exemplifies this. Lenin did not intend to rule the nation solely. He had a party in place and collaborators. In contrast, Stalin ran off Trotsky (Snowball in the book) and imprisoned or killed anyone who opposed him. At the height of his dictatorship, he controlled everyone and became more and more corrupt because of the absolutism of his "reign."
That being said, I have to disagree with Post 3 because so far, U.S. Presidents have never had absolute power and cannot Constitutionally obtain it. While one could argue that a President could be absolutely corrupt, the quote implies that absolute power must be obtained in order to be absolutely corrupt. I think that it would be difficult to prove that any U.S. President was completely corrupt. While many have been involved in scandals and have abused their power, they still seemed to try to do good for America at different points in their Presidencies (i.e., Richard Nixon was able to pull troops out of Vietnam despite his corruption).
I agree with Pohnpei in the previous post. The more power has person has, the more likely he is to abuse it. Definitely reminds me of one of our most recent past Presidents.
The idea behind this quote is that the more power you have, the more likely you are to abuse it. If you have some power, you get to be somewhat power-hungry and you are somewhat likely use your power wrongly. If you have absolute power, you completely lose any morals you have and you abuse your power to a huge extent.
Given that this is in the Animal Farm book, think of Napoleon. At first he's a little corrupt, taking the milk for the pigs, for example. But as he gets more power he gets worse and worse until he's carrying the whip around and selling Boxer's carcass.