Answer Description
The three types of authentication factors are something you know, something you have, and something you are. In this case 2 factors are used: 1. The user must know the username and password. 2. The user must have a valid client side certificate.
Wikipedia
Multi-factor authentication (MFA encompassing Two-factor authentication or 2FA, along with similar terms) is an electronic authentication method in which a computer user is granted access to a website or application only after successfully presenting two or more pieces of evidence (or factors) to an authentication mechanism: knowledge (something only the user knows), possession (something only the user has), and inherence (something only the user is) MFA protects the user from an unknown person trying to access their data such as personal ID details or financial assets A third-party authenticator (TPA) app enables two-factor authentication, usually by showing a randomly-generated and constantly refreshing code to use for authentication
Multi-factor_authentication - Wikipedia, the free encyclopedia