Google's 2FA authenticator is an app that adds a second layer of security which requires a 6 digit code along side your password to log into your account.
This additional security feature helps prevent your account from being hacked since they won't be able to log into your account with just your password alone. They will also need to have access to your phone which makes it very difficult for unwanted individuals to access your account.
The 2FA Authenticator app on your phone generates a 6 digit code that changes every 30 seconds. This code is what you enter in the field below where you enter your password.
Please check this link for more information Google 2FA - How does it works?
If you want to learn how to set up your 2FA Authentication for your account, please us this link.