What it is
A mechanical watch is powered by a coiled mainspring in a barrel; energy comes from winding (manual) or a rotor (automatic). The mainspring slowly unwinds, releasing energy to the gear train. Power reserve is the time from full wind to complete unwind. Once the mainspring is unwound, the watch stops; you'll need to wind it again or move the rotor (wear it) to restart.
Modern reserves
Through the 1990s, ~40-44 hours was standard (the ETA 2824 is 40 hours). The 2010s saw a weekend-proof movement race; brands extended reserves so a watch left off Friday morning would still be running Monday morning (60+ hours). Modern Rolex Cal. 32xx is 70 hours; Omega Cal. 8800/8900 is 55-60 hours; Seiko 8R29 is 55 hours; Lange Datograph Up/Down is 60 hours.
Twin and triple barrel
Twin-barrel architecture stacks two mainsprings in series, doubling reserve. H. Moser & Cie. Endeavour Concept (~3 days), IWC Big Pilot (8 days = 192h), Breitling Premier B25 (336 hours = 14 days). Triple-barrel mechanisms exist but are rare. The trade-off: more barrels add movement thickness; very long reserves require bigger barrels which require thicker cases.
What you actually need
40 hours: minimum acceptable; the watch stops over a weekend if you don't wear it. ~70 hours: weekend-proof; comes back from being off Friday → Monday morning still running. 5+ days: 'desk diver' reserve; the watch survives a multi-day work trip without re-setting. Power-reserve indicator: a sub-dial showing how much reserve is left. Helpful on manual watches; mostly decorative on automatics.