Two-factor authentication is an additional security measure for your account. In addition to your usual login and password, you will be asked to enter a security code which is generated by a third-party mobile application (such as Authy, Google Authenticator or Microsoft Authenticator).
To enable two-factor authentication go to Settings in the Panel, scroll down until you see "2-factor authentication", and then click "Enable". Download application for your mobile OS and scan QR-code shown on the screen. Finally, enter the security code generated by the app in the required text box ("Enter token from your phone").