It's allegory about communism
Animal Farm is a satirical novel (which can also be understood as a modern fable or allegory) by George Orwell, ostensibly about a group of animals who oust the humans from the farm they live on and run it themselves, only to have it corrupted into a brutal tyranny on its own. It was written during World War II and published in 1945, although it was not widely successful until the late 1950s.
Animal Farm is a thinly veiled critique and satire of Soviet totalitarianism. Many events in the book are based on ones from the Soviet Union during the Stalin era. (For example, the character Snowball, who is expelled from the Farm by Napoleon, is clearly modeled on Trotsky.) George Orwell, though a leftist he was for many years a member of the Independent Labour Party was a critic of Stalin, and suspicious of Moscow-directed communism after his experiences in the Spanish Civil War.