An Authenticator App is a app that you install on a smart device, in most cases a mobile smart phone, that turns your smart device into a electronic authorization device for your on-line accounts. It does not replace the password sign-in part of the authentication, rather, the Authenticator App acts as a secondary conformation that -you- are in-fact the person who is signing in with your associated password. Many sites are including this feature as part of their 2-factor authentication processes. When you enable this feature, after you sign-in with your user-name and password, you are then asked to enter an unique Security Key which is generated by the Authenticator App. The Security Key changes every 30 seconds to prevent hacking of the key itself. You can also check out my article “protecting your smart phone from be hacked” if you want to learn more about securing your Authenticator Device.
Please note that if you lose your smart device, or do not have it with you when you attempt to sign into a service that has this feature enabled, you will be prevented from signing in unless you use a backup method. Every service has different backup methods. Its been my experience that backup methods are a security vulnerability and I recommend that you only use the most strict backup methods possible: Backup codes are better than security questions.
Below are some Authenticator Apps. Most Authenticator Apps only work on one device. But having a backup device might be preferable in case you lose access to your primary. I recommend Authy for the following reasons: 1. Its not tied to a parent conglomerate that has a history of weakening privacy or security for financial incentives and 2. You can install Authy on multipole smart devices and share you codes across those devices. This ability acts as a backup in case of a loss of a primary device.