It doesn’t DO anything. That’s the beauty of it.
My original thought was something like a clock - sophisticated, but not so complex as to be impossible. I really think that most of the devices in science fiction stories would never be reliable over the long term. Think about the MTBF for a personal computer, and then extrapolate it to a “suspended animation” type device. I just don’t think that a hugely complicated piece of electronic equipment could last centuries, unless you have robots with AI that could fix it and themselves…
The robots would have to be able to fix themselves as well, since they shouldn’t last any longer then the machines they are repairing.