Two-factor authentication

Open the account menu (your avatar at the top right) and choose Security; its 2FA badge shows whether two-factor authentication is on or off. In the Two-factor authentication card press Turn on two-factor authentication, scan the QR code with your authentication app (Google Authenticator, Authy, or similar) or copy the secret key into the app manually, then enter the generated code to confirm and activate 2FA on your account.

During setup you receive a recovery code. Save it in a safe place: if you cannot access your mobile device, entering a recovery code is the way to verify your identity and get back into your account.