What is Low Power Mode in iOS 9 and how do I enable it?

Apple introduced Low Power Mode with iOS 9 to give you some extra time before your phone dies. It works by reducing various background activities that drain your battery such as Mail push, automatic downloads, background app refresh and animated effects. It also reduces the iPhone’s CPU performance. Once your iPhone gets to 20% battery, it will ask you if you want to switch to Low Power Mode (and it will prompt you again at 10%). If you turn it on at this point it will supposedly give you an extra hour of life before you have to charge it. Once you do charge it, Low Power Mode will automatically turn off once your battery is above 20%.