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: