Multi-factor authentication describes the necessity for using more than 1 token as a proof of identity. As an example, when a user logs on to to a digital platform the basic means of authentication are user name and password. 

In addition to the password (single authentication), the user can be asked for a physical token (RFID-key, ID-card, ...). This can also be a mobile phone, an authenticator app token, a SecurID or Digipass token, or biometric (fingerprint, facial recognition, ...) elements.

In security terminology this related to the concept on assuring someone's identity by something the user knows (password) and something he/she has (physical token). Additional layers can be built into this concept in order to further improve and strenghten the security levels. 

When proving someone's identity at the front gate on the basis of an ID-card, Driver License or verifiable photo-ID, it can be enhanced with a log into the system that the person has reached the premise. With his personal RFID-token, he will be able to access his office. Meanwhile video surveillance camera's might have identified him in the building. Finally when logging on to his system on the network, he can be asked for an authentication code coming from his company mobile phone. 

These additional levels of authentication harden the security and can be continuously expanded, depending on the security levels required.