A Bundle ID is a tool Apple uses to identify individual apps.
An app cannot be submitted to iTunes Connect before it has been assigned a Bundle ID — you need to create a Bundle ID before submitting your app to iTunes Connect.

You can change your Bundle ID on iTunes Connect until your app has been approved by Apple and is available on the App Store.
Once your app is approved and available for download on the Apple App Store, your Bundle ID cannot be changed.

If you need to find the Bundle ID of an app already approved by the Apple App Store, click here.