For Android apps, you simply need to enter a unique identifier for your app, of the form

com.myapp.myuniqueapp

It must start with com and be different for every app. If you use the same identifier for a new app, and you install it on a device that has a previous app with the same identifier, your app will crash.

Your identifier is used when you publish to the Google Play store. From the Developer instructions for Android apps:

Every Android app has a unique application ID that looks like a Java package name, such as com.example.myapp. This ID uniquely identifies your app on the device and in Google Play 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 APK—if you change the application ID, Google Play Store treats the APK as a completely different app. So once you publish your app, you should never change the application ID.