When you enable Two-Factor Authentication for your Recurly account, you are required to enter your password and a second authentication code when you login to Recurly. You will be required to do this if it has been more than 14 days since your last session, or if you log in from an unrecognized device. The authentication code can be delivered to you either by SMS or via the Authy app that you can download to your mobile device, laptop or desktop computer.
Two-Factor Authentication is associated with your user profile in Recurly. So if you have access to more than 1 Recurly site (e.g. a sandbox site and a production site) with the same email address, if you enable two-factor authentication, it will apply regardless of which site you are logging in to.