One-handed mode reduces the display size on the Samsung device so that the device can be used with one hand. When one-handed mode is activated, the view can be moved to the left or right side as desired. The display size (the window) can also be adjusted.

To start the one-handed mode, you can either start the gesture (from the Home button down) or the button (double-tap on the Home button), but before that, the one-handed mode has to be activated in the settings for advanced functions.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Advanced features
Step 3:  Tap on  One-handed mode
Step 4:  Enable or disable  Use One-handed mode
  1. Open the Settings
  2. Tap on Advanced features
  3. Tap on One-handed mode
  4. Enable or disable Use One-handed mode

Samsung Instructions