This quote, of course, is from 1984, Orwell's other and equally famous dystopian novel. It is found when O'Brien explains the purpose of power to Winston during his torture and re-education at the Ministry of Love near the end of the book, and it is intended to show that dictatorships are never really established in order to bring about some sort of social change (as was argued by Communists, for example.) In reality, dictatorships were all about power, and O'Brien is telling Winston that the Party will continue to increase its power, in increasingly subtle ways, over the people.
So what does this have to do with Animal Farm? As the book goes on, we see that the revolution the animals have carried out, motivated initially by the teachings of Old Major and the horrors of human rule, has become perverted by its leaders, the pigs. Napoleon and the other pigs are using their leadership of Animal Farm to enrich and arrogate power to themselves, and by the end of the book they are scarcely different from their human rulers at all. They have, through a series of lies perpetuated by Squealer, continued to justify the dictatorship they have established by referring to the original ideals of Animalism, but the reader sees that it is all a sham. The animals are no better off than they were under Jones, and they can envision a sad future not unlike the grim vision described in the quote from 1984.
At the same time, the critique of power offered in Animal Farm is perhaps a little less nuanced than 1984. The pigs are motivated by naked self-interest. They do not wield power just for the sake of power, but rather enrich themselves through dealings with the humans. It is unclear that the Party has such interests (indeed O'Brien says, in not so many words, that they do not.) Their goal is to control all of human behavior for the sake of power itself, whereas Animal Farm portrays the pigs as simply greedy and self-interested, if equally ruthless and brutal. Exploring these similarities and differences would make for a strong essay on the theme of power revealed in the quote, and any strong thesis should focus squarely on this theme.