Two-factor authentication, TFA or 2FA, is a more secure way of sign-in  that requires more than just a password. Without TFA, the account is less secure, because the password is the only security factor (layer of the security) in that case. TFA provides extra security layer by requiring additional information to sign in. This additional information is a code that’s generated on your smartphone or tablet. This code is generated by special security applications (for example, Google Authenticator or any other from your App Store or Playmarket).  In case of Deribit, two factors of security include: something you know (a unique username and password) and something you have (a smartphone or tablet) to approve authentication requests.

Deribit TFA code is the so called Time-based One-Time Passcode (TOTP). TOTP is a variant of a hash-based message authentication code, see description of the standard RFC 6238. It combines a secret key with the current timestamp using a cryptographic hash function to generate a one-time password. The generated TOTP is valid in 30-second intervals, so you may need to check and adjust properly your system clock and time zone on your smartphone or tablet.

Further reading: Google Authenticator (iOS)Google Authenticator (Android)TFA in Wikipedia