Power reserve
Power reserve is the number of hours/days that a fully excited spring can give energy before the watch has to be rewound.
Power reserve is also called 'Réserve de Marche'.
Generally, a fully wound up spring has about 36-42 hours before it stops, but there are also timepieces that have a power reserve up to three or more days.
The power reserve indicator is a complication of the watch that indicates how many hours or days the watch still has energy before it has to be wound up again.