Multi-factor authentication (MFA) is an industry term that describes a security process that requires you to provide more than one form of identification or authentication.
In other words, a multi-factor authentication system is one where you need to provide two or more pieces of information to prove your identity.
The most common form of multi-factor authentication is using a physical token, such as a key fob, which generates a one-time password (OTP) every time you log in. This is often referred to as something you have as well as something you know (i.e. your password).
If someone steals your password and uses it to log in to your account, they can't get in because they don't have the physical token — and vice versa.
The second form of multi-factor authentication involves something called biometrics — for example, scanning your fingerprint or retina before granting access to an account.