If you opt into Login Approvals, you will have to "Confirm your phone". You would receive a text message with a specific code to be entered into the system. If you don't want to do this every single time you sign on, you can designate your smartphone, PC, or tablet as a trusted device and Facebook will allow you to sign in without the text code.
Should an attempt to login from an unrecognized device happen, it would not be allowed.
If we ever see a login from an unrecognized device, you'll be notified upon your next login and asked to verify the attempted account access. If you don’t recognize this login, you'll be able to change your password with the knowledge that while some one else may have known your login credentials, they were unable to access your account and cause any harm.
If you lose or forget your phone, meanwhile, you can still authorize your account, provided you're accessing it from a trusted device.
Facebook acknowledged that it's sometimes difficult to balance security and usability.
Introducing Login Approvals [via]