two-factor authentication.

https://www.facebook.com/about/basics/stay-safe-and-secure/login-approvals#16

More and tap Account Settings.
Tap Security and Login.
Tap Use 
Tap Two Factor Authentication.
 set up login
Enter your mobile phone number.
Enter your code.
 Confirm.
Close.

fb
Enter your code and tap Continue.
notification from two-factor authentication
log in