[Blacklight]

This is a topic I know a LOT about as 2001 is my favorite movie ever.

I read the books. According to 2001, the Monoliths represented an alien technology or intelligence WAY beyond our understanding. The one on the Earth kick started human evolution. The one on the moon waited there for humans to gain the technology to find it and sent the message that the humans had reached that ability to the one in orbit around Saturn (It was Saturn, not Jupiter in the book). The giant one in orbit around Saturn (or Jupiter in the movies) was a star gate to a higher plane of being. Kind of the final evolutionary step.. and also... after Dave Bowman was transformed by his passing through the stargate, the technology and intelligence of whaterver created the monoliths were still beyond his understanding.
So the monoliths pretty much represent some kind of unknown HIGHLY advanced intelligence. They could be machines. They could be the new form of the race that created them (Dave Bowman saw the entire evolution of the alien race that spawned them when he entered the stargate. The movie kind of glossed this over with a psychadelic lightshow). They are some strange kind of unknown that mankind will never be able to grasp.

The monolith from the moon, after tons of experiments were done on it, was cemented in the ground standing in front of the United Nations building as a symbol to say "Humanity needs to be united because we aren't alone" and it worked. There was pretty much world peace after the events of 2010. Also, they discovered the monolith buried in Africa as well in an archeologica dig.

We will never truely know exactly what the monoliths are. Arthur C. Clark never wanted us to know. They are beyond any understanding that we may ever have.