Two-factor authentication

1. Go to Account → Security and choose Enable two-factor authentication.
2. Scan the QR code with an authenticator app (Google Authenticator, Authy, 1Password…), or enter the code shown manually.
3. Enter the 6-digit code to confirm.

From then on you'll be asked for a code at sign-in. You can Disable it from the same page if you need to. Enabling 2FA also signs out your other sessions.