I am assuming that you have not verified your app with Google.
An unverified app is an app that requests sensitive or restricted OAuth but hasn’t gone through the Google verification process. Users of unverified apps might get warning based on the OAuth scopes you’re using.
This is to protect users and their data from deceptive apps.
A published app is an app what has been deployed whether it is verified or not. The owner of the app can access it without any issues, but when a user tries to access it, it won’t work because its not verified.
The link that @ceejayoz is very useful and has plenty of information.
An app needs verification before you launch a user-facing app. You can continue to build and test your app while waiting to complete verification
Apps that don’t require verification:
1- Apps in development.
2-OAuth-Based plugins
3-Internal apps
To verify an app, you have to follow the steps in the link [1].
[1] https://support.google.com/cloud/answer/7454865#verification