Low power mode activates automatically when your iPhone's battery reaches 20%. 

Low Power Mode temporarily reduces screen brightness, disables automatic mail fetching, and pauses background activities.

 You can manually activate Low Power Mode in the Settings app. Navigate Settings > Battery, then tap the "Low Power Mode" toggle.

 Low Power Mode shuts off automatically, but you can manually change your screen brightness, disable background activities, and other similar things to save power.

Your iPhone has a "Low Power Mode", which you'll be prompted to activate it when your phone reaches 20% battery. 

You can also enable Low Power Mode before that point to stretch your battery life further. Here's how it works.

For more such interesting stuff, click on the link given below