2001 A Space Odyssey - ending

Dave (old guy) is near death. In front of him monolith shows up. The monolith is somekind of step to a higher evolution level made by aliens. So then he is reincarnated in advanced spirit and new body form, a child, which could be a metaphor for purity, harmony and love. And now he watches the earth, actually means he is among us, still having bond with humanity.