He made it all the way to Jupiter, and figured he might as well check out the monolith despite the disaster of the mission so far. He took the pod to check it out. He was then taken through a portal in the monolith to the home system of the aliens that created the monolith. There, he was put in a simulation as best as could be created based on TV signals from earth. The simulation was of a hotel room in a particular TV show. The alien machinery then downloaded his mind, hence the aging sequence. He was then inserted into the substrate of spacetime, allowing him to move around the galaxy like an omnipotent god. He decided to go back to earth to help out humanity, but he was still very new to this upgrade, so he was like a baby, hence the fetus over earth, which was just metaphorical. His upgrade was the next step for humanity, just as the upgrade for the man-apes was, giving them the ability to use tools.
This was all explained in very plain English in the book.
This was all explained in very plain English in the book.