An authentication app such as Google Authenticator is an OTP (one-time-password) app. That allows users to utilize MFA (multi-factor authentication) or 2FA(two factor authentication) in order to access an account or profile.
Typically, once the password is entered, the service or login will prompt the user to access the app in order to access the OTP via text or email. Then, the user enters the OTP into the service and is granted access.
One-time passwords satisfy the possession (something you physically have with you) factor offering added security to outdated SSO (single-sign on) username and password logins.
OTP’s are additionally available via SMS & email, however (a word of caution). As they are one of the weakest forms of 2FA security & UX wise. I can’t tell you how many times I go to utilize a OTP and my phone is dead, in another room. And how common simjacking & how prevalent other hacks on this method have become.
Some of the most popular OTP apps include Google Authenticator, TOTP Authenticator, AUTHY. The advantage of GA and similar systems is that there is nothing for a hacker to intercept. An easier way to implement 2-factor authentication is for the server to text you a secret number to your phone. A hacker can intercept this text and get into your account. It’s called a SIM Swap attack. With GA, nothing is sent to you, so there’s nothing for the hacker to intercept.