An automatic watch means that it is self-winding. The springs in an automatic watch are set to move with the natural movement of your wrist, so the watch stays wound when worn. An automatic watch can also be wound manually.
To restart a watch that stops after roughly one to two days of not being worn, put it on your wrist. After natural movements of the wrist, the watch will restart and continue to wind throughout the day.
To keep the watch wound, use an automatic watch winder. Put the watch in a case that moves periodically to keep the watch wound. It is better to keep a watch wound than always letting it stop and having to wind it. A watch winder is particularly useful if the watch has more complicated movements, like a calendar, because the date does not need to be reset after the watch stops.
A watch winder starts at around $60.
Manually wind the watch. Certain automatic watches, like a Rolex, can be wound manually, but many, like a Seiko, do not have this option. Each watch has a different method of winding. For a Rolex, first unscrew the crown until the pin pops out. Then twist the crown clockwise. If you put the watch to your ear, you will hear a fast "ticking" sound. Turn the crown no more than 15 full rotations. A watch can be over-wound, so it is important to be accurate when manually winding a watch. Some watches do not have a crown that pops out and some crowns need to be pulled out past two notches. It depends on the brand and style of watch.