Google Authenticator is a mobile application made by Google that implements two-step verification. Authenticator provides a six to eight digit one-time password which users must provide in addition to their username and password to log into an application.
Click the link below that most fits your role to learn more:
Google Authenticator for Administrators - this page explains how to enable/disable using Google Authenticator, how to reset accounts, supported devices and customization.
Google Authenticator for Developers - this page explains how to enable Google Authenticator and details on how to configure it to meet your needs.
Google Authenticator for Users - this page is for users logging into your application using Google Authenticator. It also has troubleshooting tips for any issues they may encounter.