In the Account Settings, users can opt to get a special password code delivered in text to their mobile phone to strengthen login authentication. You must register your mobile phone with Facebook for this feature. This is documented below.
QUOTE: Facebook is now offering a new feature called “Login Approvals“. I call it part-time two-factor authentication mechanism. Andrew Song of Facebook states: “Login approvals is a Two Factor Authentication system that requires you to enter a code we send to your mobile phone via text message whenever you log into Facebook from a new or unrecognized computer.” I have downgraded it to “part-time” because once you have approved the browser instance you are using to login to daily, it does not require execution of the second authentication until you have removed it from the list. I clarify “browser” because you will be forced to re-auth from a different browser.