The Planet of the Apes 1968 ending reveals that the apes' planet is Earth in the far future, ruined by human war. Taylor and Nova ride along a beach after leaving Dr. Zaius behind, and they come upon the wreck of the Statue of Liberty, half buried in the sand. Taylor understands what the whole film has been hiding, and he collapses in rage and grief and curses humanity.
How does the reveal work? In my opinion, it works because the film plays fair. The astronauts' instruments already suggested about two thousand years had passed on Earth, and the crash landing in a lake gives no clue about the sky or the stars. Zaius had just shown Taylor a cave of evidence for an older human civilization and then destroyed it. Zaius admitting he has long known the truth means the apes' official history is a managed story, and his warning not to look for answers is the last piece. Taylor goes to find the answer anyway, and the answer is the beach.
It could be argued that the staging matters as much as the idea. Rod Serling's twist ending survived the rewrite by Michael Wilson that made the apes more primitive, which suggests the filmmakers treated the final image as the fixed point of the whole story. Serling's first script imagined an advanced ape society that was too expensive to build, so the cheaper, rougher ape village at Malibu Creek and the empty shoreline at Malibu did real narrative work: a quiet beach, a ride on horseback and one wide shot carry the reveal without any exposition.
The ending also changes how the earlier scenes read. Taylor's speech to the apes after he regains his voice, and the hearing where he learns that Landon has been lobotomized, are scenes about humans being denied a mind. Once the beach arrives, the film turns that argument back on Taylor's own species, since the apes did not ruin the planet and humans did. Zaius is cruel, but he is also the one character who has seen the proof and chosen to hide it.
The sequels then have to answer the questions the beach raises. Beneath the Planet of the Apes sends a second crew after Taylor, who vanishes through a rock wall with Nova left behind wearing his dog tags. Escape from the Planet of the Apes has Cornelius, Zira and Dr. Milo repair Taylor's ship and flee to 1973 Los Angeles, and Conquest of the Planet of the Apes opens in 1991 after a plague has killed dogs and cats and people have turned apes into pets and then slave labor. The modern run starting with Rise of the Planet of the Apes explains the fall differently, through a lab drug and Caesar, so the 1968 twist is the only version where a single image delivers the whole backstory. If you want the chronology in full, the Planet of the Apes timeline lays it out.
Is the film racist? That question comes up often, and I would answer it as opinion rather than fact. The apes sort themselves by species, with orangutans governing, chimpanzees working as scientists and gorillas acting as soldiers and hunters, and they treat mute humans as animals to be caged or killed. In my opinion the film uses that arrangement to put prejudice on screen and then condemns it, since the final image blames human violence and the apes' certainty about their own superiority is what sends Taylor into the Forbidden Zone. Perhaps a viewer will still read the allegory differently, and the film does not settle the argument for anyone.