Natively will automatically generate a bundle identifier for your app, otherwise, you can create your own.
A bundle ID, otherwise, known as a package in Android, is the unique identifier for all Android apps. It needs to be unique, as when you upload it to Google Play it identifies and publishes your app using the package name as the unique app identification.
Really it is the only thing that is necessary to identify your app, and generally, it has 3 parts:
Where "example" is generally the company/publisher's name, and "myapp" is the app name.