A Bundle ID is created automatically when you create an app. The Bundle ID is the unique identifier of the iOS (Apple) version of your app.

For Android (Google) apps the unique identifier is called Package Name. 

You can change your Bundle ID/Package Name if you need to update an existing app which has another Bundle ID or Package Name for example. Otherwise, it’s strongly advised that you do NOT change them.

Changing your Bundle ID and/or Package Name will prevent your app from being updated in the App Stores. Instead of your users having your existing app on their phone updated, they will have to download the app again.