iOS Bundle ID
A Bundle ID is a unique identifier of your app on iOS and MacOS. iOS and MacOS use it to recognize updates to your app. The identifier must be unique for your app!
When creating a new project, XCode forces you to define a product name for your project and generates a bundle ID from the product name and your team's domain. This is usually in the form of com.<TEAM_NAME>.<PRODUCT_NAME>. For example, in the case of Blink ID and Microblink the bundle ID is com.microblink.blinkid: https://itunes.apple.com/us/app/blinkid/id1258136557?mt=8
The bundle ID can be changed at any point. You can view and change it in XCode in the General tab. To access the tab simply click on the project in the Project Navigator.
After the app is submitted to iTunes Connect it is registered with the bundle ID as it's unique identifier. Any updates to the app will be recognized as updates. Changing the bundle ID at that point would require the app to be re-submitted to iTunes Connect as an entirely new app. Any changes would be recognized as updates only in the new app with the new bundle identifier.
Android Package name
The package name is a unique name to identify a specific app. Generally, the package name of an app is in the format domain.company.application , but it’s completely up to the app’s developer to choose the name. The domain portion is the domain extension, like .com or .org , used by the developer of the app.
Once you publish your application under its manifest package name, this is the unique identity of the application forever more. Switching to a different name results in an entirely new application, one that can’t be installed as an update to the existing application.
The sample of the package name for the Micoblink's BlinkID demo app is: com.microblink.blinkidapp