The AE-35 unit is the dingus that stops working because Hal has a glitch. It's the technical gizmo that controls Discovery's communication with earth. With it, the astronauts can get info from mission control and chat with their families (on a time delay). Without it they're on their own, in the deep silence of space, with only a homicidal computer for company.
The AE-35 can be seen as just an example of space-hardware; it's got an official sounding name, it does official sounding things—it's mostly an excuse for the novel to tell you seriously "This is a small but vital component of the communication system," (21.34) and spout other jargony goodness that makes you feel like you're reading about serious space science. It's a genre gimmick—part of the furniture of sci-fi.
It's important, though, that the AE-35's job is to connect the Discovery to earth. This isn't just any piece of technological detritus—it's technological detritus that ties Bowman and Poole to their home. The most human part of the ship, then, is in some sense this little gadget. Remember, what makes the man-apes human, or starts them towards being human, is their ability to use tools. The AE-35 unit is a tool too—and tools are what makes people into people, just as, onboard ship, the AE-35 is what ties Bowman and Poole to the human race.
But Hal is also a technology—and it's Hal that's sabotaging the AE-35. So you could argue too that tools, or technology, disconnects people from humanity.
Or, in this case, it's a back and forth. Is Bowman star baby more human than human as the alien technology evolves him, or is he no longer human and something else instead? Overall, the novel is enthusiastic about the possibilities of technology and progress—but every so often there's a doubt. The glitchy AE-35 symbolizes both the enthusiasm and the uncertainty.
And of course the fact that sometimes all those awesome tools just doesn't work very well.