Two Factor Authentication is a security measure to protect against unwanted access to your account. You (or anyone else) can not sign into your Passpack account without it. If you opt for the Yubikey, please make sure you have at least 2, otherwise if you loose your Yubikey, you will forever be locked out of your account.
(1) Go to Settings > Two Factor Authentication.
(2) Select one of the two options. If you choose Yubico Yubikey USB card you must have, or purchase a Yubikey. Choose One Time Passwords sent via email for a free alternative, though not quite as high-security.
(3) Press Continue and follow the on-screen instructions to finish the process..