They’re supposed to be intelligent supercomputer machines that were created by a definitely humanoid and possibly human race that existed 1 billion years ago... a race who had reached they’re singularity by 10 fold.
A race who also had the technology to download their minds Mechanist Religion style probably similar to the monoliths... even greater than that, their minds could roam free which is stupid... because well not only is Mechanism stupid but their claim would be that they became a Vitalist lifeform, which is also stupid as this would mean that they’re in fact dead aliens or dead humans who can now communicate with the dead aliens or dead humans that died before their singularity happened.
I think the secret is that the monoliths actually were the first born alien/human race disguising themselves as the superHAL’s of their time.
As to whether they were actually human... the evidence is actually strong for this, because they never found any other intelligent humanoid race apart from humans in their entire search... which would mean that only the first born and the second born (humans) ever existed.
Or, the question could be - what were the Monoliths in 2001: A Space Odyssey? Because they were the same ones.
I haven't read extensively what Arthur C. Clarke may have claimed elsewhere about them, but in the films they are still enigmatic.
"I'm not smart so much as I am not dumb." - Harlan Ellison
The intro of 2001 sums most of it up it up with the ape-men scene.
They were made to find life, serve as observation devices and to promote evolution if the life they found was deemed worthy of attaining higher consciousness.
The Quatermass and The Pit (aka 5 Millions Years to Earth) deals with a similar concept of human evolution being due extraterrestrial forces, but not in a benign manner as the Monoliths in 2001/10. Decent 60's horror flick it is, though I have a fondness for the cheesiness of Hammer Films.
Basically they are swiss army knives. In the books they were the tools of the Alien intelligence and how it interacted with us. They were able to teach, evolve and even turn Jupiter into a sun by adding more mass so fusion could start. All these worlds are yours except etc.