Google Authenticator is an app by Google that helps your verification process in two steps. It’s really easy to use: every time you try to identify yourself with your Google account from a smartphone, it’ll ask you to introduce a unique code generated at that moment by Google Authenticator. These codes expire after a certain amount of time, of course.
Although you’ll generally use Google Authenticator in order to identify yourself from Google services, there are other apps and services that also use it perform their two-step verification process. This is why Google Authenticator is one of those apps that’s always helpful to have around.