Two-factor authentication (2FA) is a sign-in or approval check that asks for two pieces of evidence from different categories, usually something you know, such as a password, and something you have, such as a phone or security key. It is the most common form of multi-factor authentication, where the full explanation sits.
Not all second factors are equally strong. A one-time password sent by text message is better than a password alone but can be stolen through SIM swapping or fake login pages, whereas authenticator apps and hardware security keys resist these attacks better. In European payments, strong customer authentication under PSD2 requires a two-factor check for most online payments. 2FA is one part of wider authentication, which proves who a user is.