MFA is Multifactor Authentication; it is a means of user authentication. In general, there are three ways to authenticate a user.  First is something the user knows a password or the answers to a series of questions.  Second is something that the user is, for example, a fingerprint, facial recognition.  The third is something the user has as a token, card, or phone. In many applications, MFA is used in addition to a password to validate the individual trying to gain access; passwords can be compromised. MFA is a second step in the assurance process.
MFA is easy to use and is highly recommended as a means to validate users for all systems.  However, all MFA techniques are subject to hack or compromise, which calls for a defense-in-depth approach to system security.