Two-factor authentication adds a second verification step, such as a code or security key, after a password.
Two different proofs
2FA combines a password with a second factor, such as an authenticator code, hardware key, or approved device. Stealing the password alone should not be enough to sign in.
Factor choice changes protection
One-time codes are stronger than passwords but can be phished. Security keys resist many fake-login attacks, while recovery procedures determine what happens when a device is lost.