In the book by Arthur C. Clarke, Dave Bowman lands on a giant monolith orbiting jupiter, a much bigger version of what was found on the moon (which taught early humans how to use tools). After being transported through an interdimensional wormhole, he ends up in a strange looking room, where he is aged by the advanced civilization (the creators of the monolith) very quickly. In the movie it seems as if he's looking at himself at older ages, but it's just different stages of life. At the end of his life a monolith appears, and Dave becomes a "starchild" or messiah if you will, a fetus that floats towards the earth, presumably to do great things. If memory serves correctly, the last line in the book is something like, "he didn't know what he was going to do, but he would think of something."