1. Who made it?
2. What did they make it for? We think it was used to predict eclipses. That's in dispute.
It's agreed that it's ancient - something on order of 2100+ years old. The conundrum is that it contains technology that was believed to be out of reach of humans from that time period. The article is great. It contains the following video showing how it works using a working model.
THE ANTIKYTHERA MECHANISM WAS EVEN MORE RAD THAN WE THOUGHT | Gamma Squad