Two-factor authentication (2FA) is an access security method that requires two different forms of authentication. Only when you have passed two factors are you allowed access to your account.
To try and avoid any hackers from accessing your account, the addition of a second separate method of verification makes it much harder.
The first factor is usually a secure password.
The second factor is usually one of the following:
Mobile Phone Number; they will send your registered phone a text message with a code for you to enter.
Voice Call to Phone; an automated voice will call your registered phone with a code for you to enter.
E-Mail; A time-limited code is sent to your registered email address for you to enter
Fingerprint Biometrics; many modern phones have a fingerprint reader built into the phone
Facial Biometrics; Modern phones can be used to scan your face in order to create a 3D face map