Arguably, the definitive science fiction movie, but also the most misunderstood as it seems to be so ambiguous, it's up to the viewer to take what they want from it. Some may choose to mull over the visual poetry as they may see fit, while others might just time their hallucinatory aids so they can enjoy the trip. Both legally and morally, I would have to recommend the former.
For me, the premise of Stanley Kubrick's masterpiece is that an unknown being (represented in this film by a mysterious black monolith) has been observing life on Earth since the very beginning, and only intervening on rare occasions to evolve human intelligence.
The movie is split into two main sections: the first being 'The Dawn Of Man', where primates are fighting amongst themselves, and after one of the apemen discover the monolith, he happens upon the concept of tools & weapons, using a bone as an implement for battle.
The second section of the film follows a group of astronauts on a mission to Jupiter, while their spaceships central computer, HAL, tries to take control of the mission for unknown reason.
The final half-hour is where you can choose to dissect the meaning of it all/experience the psychedelia (depending on how much you've been smoking), but for me the message is as follows:
The technologies we encounter through human history, from the first rudimentary tool to the super computers that navigate the universe, are based on the same intellectual level- they can be used for good or evil and the choice is up to the user.
Kubrick's masterpiece of science fiction has become hugely influential and much imitated since it's 1968 premiere, and though it's still very much misunderstood, not many would be able to argue that it's a masterpiece of filmmaking excellence.