What was the meaning of 2001 A Space Odyssey?

The most common interpretation is that the meaning is all about how man abuses its tools. In the beginning, millions of years ago, when the monkey hit the other monkey with its tool (the bone) was the first step. The black monolith (rectangle) represents man's challenge to be able to actually get smarter and/or progress as we grow. It means evolutionary knowledge. HAL is man's machine, a computer, four million years later in 2001. Man developed machine to be as advanced as themselves. The machine defended himself and ended up harming man, just like the monkey used its bone to kill another monkey.

So, in the end, it tells us that we depend on our tools so much that they become too advanced and that they can eventually destroy us.

Lots of people actually just overlook an entire hour of the film and just say the message is "Don't trust electronics", but if you "go into it" a bit more, you'll realize that the real meaning also has to do a lot about evolution.