If your battery gets to 20% or less battery status, you'll get a prompt about it, giving you the option to enable Low Power Mode which reduces power consumption by reducing or turning off mail fetch, background app refresh, automatic downloads and some visual effects.
If you don't want this you can choose to cancel the prompt. And when you charge your phone and it reaches 80% of battery status, Low Power Mode gets deactivated.
You can also enable Low Power Mode in Settings > Battery > Low Power Mode.