What will be my domain in case of google drive api push notifications?

I'm using the Google Drive API for an android app and a web app. I want to 'watch' a folder on google drive and whenever a change happens, I want both my apps to be notified. I know Google Drive already offers this via the api. But the documentation says I have to first register my domain. What will be the domain for both of these? Also can someone guide me through the whole process? I am relatively new to backend dev and have gone through the documentation but the steps are not very clear.

Any help on this would be appreciated.