There are lot of new features available on the Moto G5 Plus, one such feature is the One button navigation. This feature says, forget buttons, use fingerprint sensor to navigate and get more screen space.
To enable this feature, go to the Moto app in your app drawer, tap Actions and tap on One button nav. Switch on the button at the top of the screen, You can also see a demo of how this feature works by tapping Show Me How at the bottom of the screen.
This feature allow you to use the fingerprint rather than the back and home buttons. To go back, swipe from right to left on the sensor, to open the recent apps swipe left to right on the sensor. To lock the screen touch and hold the sensor until you feel a short buzz. Touch and hold the sensor until you fell a long buzz to get the Google Now (when you have already unlocked your phone)
Feel free to turn off this feature if you are not comfortable in using the fingerprint sensors for navigation other than just to lock and unlock your phone. But when you start using it you will feel power of this feature, just give a try.