Package ID - is a unique name that identifies your app that can be found in your URL address.
Every Android app has a unique application ID that looks like a Java package name. This package ID uniquely identifies your app on the device and in Google Play Store or iOS app store.
If you want to upload a new version of your app, the application ID (and the certificate you sign it with) must be the same as the original artifact - if you change the application ID, Google Play Store and iOS app store treats the upload as a completely different app. So once you publish your app, you should never change the application ID.