Two-factor authentication (2FA) is a method of confirming a user's claimed identity by utilizing a combination of two different factors: 1) something they know, 2) something they have, or 3) something they are. It is used to increase user security.
One common example of two-factor authentication is the withdrawal of cash from an ATM. In this case, only the correct combination of a bank debit card (factor 1) and a PIN (something the user creates, factor 2) allows the transaction to complete.
For Alluma, Google Authenticator's six-digit password is used as a second factor in addition to user password/username. Google Authenticator is downloadable as an app and many cryptocurrency exchanges use this well-designed, free and robust service to help improve security for their users.
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Learn more about Google Authenticator