The U.S. Internal Revenue Service is asking tax professionals to enable additional forms of authentication in software that provides the option as an improved defense against hacker takeover attempts. The agency specifically refers to multi-factor authentication (MFA), which requires at least two supplementary data points besides the username/password combination to check the identity of a user. A step down from MFA is 2FA, a subset of MFA where the user needs to provide their credentials and another form of authentication, such as a code received on the phone or generated by an application.
Source: https://www.bleepingcomputer.com/news/security/the-irs-asks-tax-professionals-to-enable-multi-factor-authentication/