Don’t trust anybody who says they understood the last half hour of that film the first time they saw it and without reading the book. They’re lying.
An important thing to know is that the novel 2001 and the film 2001 were released simultaneously and to complement each other. The film wasn’t based on a bestselling book and the book wasn’t just a novelization of the movie- it was an odd arrangement. They were intentionally meant to go together. The movie makes a lot more sense after reading the book.
The most important thing in the novel perhaps is the obelisk and the hominids, which isn’t particularly easy to understand if you watch the film. The hominids are a group of beings far closer to apes than humans and incapable of much by way of deep thought. In the book the elderly leader of the hominids has just died and the group abandons his body; the new leader is his son, though it makes it clear that neither father nor son nor any of the other members would have remotely understood such a relationship and the new leader is ‘chosen’ as new leader basically because he was the fastest and meanest- nothing remotely intellectual about the process as they have only the beginnings of self awareness.
When the hominids encounter the obelisk (a many millions of years old relic from an advanced civilization that you never learn anything about) it scans and transforms them, especially the leader (who is the most ‘intelligent’ such as it is) and subtle differences begin to appear. His sounds, which were previously mainly screams of emotion like an animals, become slightly more nuanced, and though still not a rational animal he begins to be able to put pieces of separate memories and observations together (not in language but images- “there were prints like that last time we encountered wolves… and the time before that… so those prints must mean there are wolves around”). In the movie, this is when he realizes that he can use the bone (or is it a rock? I don’t remember) as a tool when he’s trying to get to the marrow in the dead animal, and then he realizes that the same tool he can use to crack dead bones can crack living bones, which enables his tribe to kill and drive off the members of a rival pack trying to get the same food.
Basically, the obelisk has maximized his genetic potential and greatly expanded his consciousness, thought processes, etc… His tribe and their descendants gain hegemony over the other hominids, and because their increased intellects enable them to get better food their brains develop which makes them make better tools and make more discoveries, etc.- essentially, the obelisk pimped their brains and greatly accelerates evolution of the species into humans by accelerating their consciousness and awareness.
Tens (or hundreds) of thousands of years later the mind has continued to evolve to such an extent that now descendants of these creatures are creating rudimentary spacecrafts (the “bone in the air becomes a Blue Danube space montage” sequence). They’ve even created artificial consciousness where there was none themselves (HAL)- a baby step towards what the obelisk has done. When the astronaut encounters the obelisk, it “jump-starts” his evolution and increases his consciousness to the degree that it did that of the first of his human like ancestors.
Or else gives him acid. I was never completely sure.
I do know that the first time I saw that movie (in a theater during a re-release in the early 80s) I was messed up for a while. I had been expecting STAR TREK or at very least simple-Jules-Verne-with-good-special-effects and it wasn’t that (though visually it was beautiful). In time I’ve come to appreciate it a lot more.
The first of the sequels,2010 (the only one filmed to date- Kubrick was so against a sequel that he destroyed many of the props and unused footage, incidentally) is also by Arthur C. Clarke but it is also far more conventional narrative form. (It’s about an expedition to find out what happened to the Discovery and you encountere HAL and Dave again.)
Trivia: Anthony Hopkins based the voice of Hannibal Lecter in part on HAL, who he regarded as (like Lecter) superior to humans and emotionless about destroying them, and as one of the most menacing voices he’d ever heard. (Accounts vary as to whether HAL was named to be “one letter ahead of IBM”, though I think currently it’s said that wasn’t the case.)