What does it mean to turn on dual authentication in Apple 7p settings?

For the sake of mobile phone security, iphone users will turn on dual identification, which can better protect the mobile phone and get it back quickly when it is lost.

1. Click "Set Double Authentication" at the bottom of the "Password and Account Security" interface.

2. Click Continue.

3. The prompt shown above appears because your current Apple ID has been logged into other devices that are not iOS9 system. Just click "Still open".

4. If there is no credit card bound to the current Apple ID, just click "Still Open".

5. Add your current mobile phone number.

6. Enter the received SMS verification code.

After the above six steps, the double authentication was successfully started, but it should be noted that more numbers must be added to the "trusted phone number". Even if the mobile phone is lost, we can quickly log in to Apple ID to change the password and use the alternate phone number to receive the verification code for operation.