An app icon is the picture people use to recognize and open an app. It is like the label on a jar: a small, memorable mark that helps you pick the right one.
The icon appears on a device's home screen or in an app store. Because it is shown at different sizes, a simple shape is usually easier to recognize than lots of tiny details.
Use the official brand artwork and follow the rules for the platform. The icon should identify the app, while its name and store description explain what it does.