The power reserve of a mechanical (and not only in an automatic) watch refers to the number of hours of operation it gives with a fully wound mainspring. This number is expressed in hours and is sometimes shown on the dial by a dedicated hand that displays the power reserve. As the watch runs and the mainspring unwinds, the power reserve gradually decreases until it reaches zero, when the watch, now without propulsion, stops.
In a manual watch, the winding is usually obtained by the rotation of the crown; in an automatic watch, this is done by the movement of the arm during our daily lives. This means that an automatic watch in regular use will have a virtually full power reserve every day, only to be reduced when not in use – like when you leave it on your bed stand for the night.