Tbf, I sincerely hope a lot of you understand the basic concept of a gearbox. Depending on how far back you go, a water wheel might be a pretty massive leap forward, and it might at least inspire other, smarter people of the time to work on theories of energy, and eventually electricity.
Honestly, you don’t even have to go that far. Just bringing back germ theory would massively advance the health and safety of ancient peoples. You might get a little bit of pushback trying to explain it to them though.