For the first time since we met the navigation bar, Android has a whole new way to get around. Android 9 Pie introduces a new series of gestures designed specifically for navigation, and they bring some wholesale changes to the way we launch, switch, and quit apps.
Here’s how to get started and master the new system:
Android Pie Gestures: How to get started
When you install Android Pie on your phone, your navigation bar will look the same as it always has, with a back, home, and overview button at the bottom go the screen. That’s because the new system is optional, at least for now. To try Gestures out, you need to turn it on. You can find the toggle in the Settings app. Open System, then Gestures, and tap Swipe up on home button. Inside you’ll find a solitary toggle with the same name. Flip it blue and the navigation bar will change to a pill-shaped home button and a thin back arrow.
from Greenbot How-To