The alien slab, the big sometimes transparent, sometimes black rectangular doohickey that turns man-apes into men and men into space babies—what is it doing there, all rectangular and ominous and omnipotent? What is your purpose, slab? Why do you torment Shmoop so?
The slab is three things all at once, in its big slabby mysterious way. First, it's connected to the aliens—it symbolizes them. Second, it's a kind of technology; it's a tool, even a computer. The Star Gate (one version of the slab) is obliquely compared to Hal; the novel says "It if had been alive, it would have felt excitement, but such an emotion was wholly beyond its powers" (38.7)—which makes you imagine for a second that the Star Gate is a kind of sentient computer, eager (like Hal) to fulfill its mission.
And third, the slab is human; it's the slab, after all, that teaches man-apes to be people, using tools and building space ships and learning to speak. Without the slab, Clarke would have you believe, you'd have no computers, no space ships, no Diet 7-Up. People would just be man-apes, not people.
So the slab wraps aliens, technology, and humans all into one big rectangular bundle. The slab tells us that the thing that makes humans human is alien technology; the most human bit of us didn't come with the earth—the unnatural tool-using bit of us. The slab is a symbol of the contradiction of being human. That contradiction being: to be human is to be part of one big, prolonged alien research project.