An Android application ID (Application ID) is a unique string that identifies your application across devices and app stores such as the Google Play Store.

The app ID is usually specified in the build.gradle file of your android project. It looks like a reverse domain name in the format com.example.myapp. Each application must have a unique identifier to avoid conflicts and ensure that the application is uniquely identified.

The app ID not only uniquely identifies your app in the app stores, but also defines the app's package name in the Android operating system. It is used to access various resources and application components such as permissions, activities, and services.

In addition, the Application ID may play a role when interacting with other services and APIs, such as ad networks or analytics tools, that require an Application ID for integration.