Using a mobile app for 2FA of a website

I've see several companies do this, but probably the most readily available example is how Google uses their GMail app on iOS to authenticate a user logging into a website.

When a user is on a website and tries to log in to a Google service, if the user has Gmail on their phone, the phone app will ask if the is you trying to log in. If you tap yes on the mobile phone, then website login is triggered to allow you to continue.

How is this accomplished? Are there any tutorials on the subject?