A watch winder is a small machine used for keeping an automatic watch (like a Rolex) wound when not being worn.
This is critical since Automatic Watches do not have batteries and rely on power from a wound up spring inside the watch. This winding is done by transfering the energy of movement into a wound spring.
A winder does the job of the movement of your wrist when you're not wearing your watch. The winder rotates the watch slowly, which causes the rotor inside to spin. The turning of the rotor inside the watch is what winds the mainspring. The mainspring is what powers the watch.
