It called Login Approval.Its a new system which is recently launched in Facebook in can further secure your Facebook account.This two step verification to the login process if you try to access facebook from an unrecognized device.It need your mobile on hand so you can get the text with code to authenticate your device.
To activate this step:
1.Login to your facebook account Account setting>Security>Login Approvals.
To activate this step:
1.Login to your facebook account Account setting>Security>Login Approvals.
2.By default its disable you have to just click on it and it will ask to confirm your phone number to start Login Approval feature.
3.After giving your phone number they will send a code to verify your mobile number,so just check ta massage and enter the code.
4.After confirming this step now its time to check it.
5.Close your browser remove all cookies from Facebook.com or use any other browser.
6.It will completely secure your account so now if anybody tries to login to your account and if he knows your password then still he will fail to login. ;)