Power reserve is how long a mechanical watch will keep running after you stop winding or wearing it. Think of it as the watch's fuel tank: once the mainspring is fully wound, the power reserve is the number of hours it can run before it winds all the way down and stops. Most modern automatics hold around 40 hours, though some go much longer.
How power reserve works
A mechanical watch stores energy in a coiled mainspring. Winding the watch — by hand, or automatically through the motion of your wrist — tightens that spring, and as it slowly unwinds it powers the watch. Power reserve simply measures how much running time a full wind buys you. When the spring runs out of tension, the watch stops until you wind it or put it back on your wrist.
Why the number matters
Power reserve matters most if you don't wear the same watch every day. A watch with a 40-hour reserve that you take off Friday night will be stopped by Sunday, needing a reset. A watch with a longer reserve — say 70 or 80 hours — can sit through a weekend and still be running Monday morning. If you rotate between several watches, a longer power reserve is a real convenience. If you wear one watch daily, your wrist keeps it wound and the number matters less.
Quartz watches don't have one
Power reserve is a mechanical-watch concept. A quartz watch runs on a battery that lasts months or years, so it doesn't wind down over a weekend the way an automatic does. If you never want to think about winding or resetting, that's one of quartz's quiet advantages.