The app icon is the icon that is visible on the home screen of your device.
App Icons on Different Devices
App icons are handled differently on iOS® and Android™ devices. Even on different Android™ devices and with different Android™ versions, the same app icon can look or behave completely differently.
With the legacy icon, Android™ allows you to use any shape and transparency on devices with Android™ 7 and earlier, as you can see in the upper right corner of the example app icon. With the new adaptive icon, required since Android™ 8, there are many icon options but no more transparency options. iOS® devices always display a square with rounded corners. Transparency is not possible, as you can see in the upper right corner of the example app icon.
|Legacy Icon for Android™ devices:
Adaptive Icon for Android™ devices: