Arthur C. Clarke once said, "If you understand 2001 completely, we failed. We wanted to raise far more questions than we answered."
The following was Kubrick's reponse to Clarke's comments. This was taken from an interview he gave to Playboy. I think the myth that it was supposed to be confusing has gone on too long.
"I don't agree with that statement of Arthur's, and I believe he made it facetiously. The very nature of the visual experience in 2001 is to give the viewer an instantaneous, visceral reaction that does not -- and should not -- require further amplification. Just speaking generally, however, I would say that there are elements in any good film that would increase the viewer's interest and appreciation on a second viewing; the momentum of a movie often prevents every stimulating detail or nuance from having a full impact the first time it's seen. The whole idea that a movie should be seen only once is an extension of our traditional conception of the film as an ephemeral entertainment rather than as a visual work of art. We don't believe we that we should hear a great piece of music only once, or see a great painting once, or even read a great book just once. But the film has until recent years been exempted from the category of art -- a situation I'm glad is finally changing."