A watch winder is a device with an electrical motor used to wind your watch when you are not wearing it. Due to the limited power reserve, automatic watches usually run down in two or three days if you stop wearing them or winding them up. The reserve depends on the watch brand and on the movement (caliber) used.
When your watch stops, you must reset time and wind it up. If your watch has additional functions (like date, moon-phase display, perpetual calendar), resetting them may take a lot of time and require certain skills. With an automatic watch winder, you can easily keep your watch wound and ready to go!