Because the title suggests, activating this setting places your machine into low energy mode. And it actually does work, supplying you with about three hours of additional battery life. In case you are fearful about your battery not making it via the day, that is the setting to activate.
Nevertheless, it achieves this by turning off or lowering mail fetch, background app refresh, automated downloads, and a few visible results.
iOS affords to activate Low Energy Mode when your battery goes beneath 20% and 10%, and can flip it off when the machine is charged to 80%.
You may as well flip this on manually, both from the Management Middle panel, or from Settings > Battery.