An automatic watch is a mechanical watch that winds itself using the natural motion of your wrist. Inside the case is a weighted rotor that swings freely as you move, and that motion winds the mainspring, which stores energy and powers the watch. No battery, no charging, just movement doing the work.
How the Self-Winding Part Actually Works
Every automatic movement has a semicircular metal weight, called a rotor, mounted on a pivot so it can spin freely inside the case. As you go about your day, your wrist motion swings that rotor, and the rotor is connected through a gear train to the mainspring. The spinning winds the mainspring tighter, storing energy the same way turning a crown by hand would. Wear the watch enough during the day and it stays wound; leave it sitting still for too long and the mainspring runs down until the watch stops.
Most automatics also have a manual winding option built in, so you can top off the power reserve by turning the crown if the watch has stopped, rather than waiting for wrist motion to do it.
Why It Matters for How You Wear the Watch
An automatic watch needs regular wear to keep running. If you have more than one watch and rotate them, an automatic sitting in a drawer for a few days will wind down and stop, and you will need to reset the time and date when you pick it up again. Some people solve this with a watch winder, a small motorized box that keeps an automatic moving when it is not being worn, though it is not a requirement, just a convenience. What you get in exchange for this extra bit of care is a watch with no battery to replace and a mechanical feel that a quartz watch does not have, along with a slight, normal variance in timekeeping since it is a mechanical device and not a quartz crystal.