Facebook has always been committed to protecting users account and information. Recently Facebook rolled out a new security feature called Login

Approval. If someone figure out your user name and password then they can easily login to your account. But, if you enable login approvals they cannot access your account. Whenever you try to login to your Facebook account from an unrecognized machine, it will send a text message to your phone with a security code. You have to enter this code in order to log in

1. Go your Facebook account
2. Then go to Account Settings option
3. Under Settings option click on Change
4. There you will see the login approvals check box, enable the check box there
5. Next a dialog box telling you about the login approvals will open and click on Next
6. Now you need to verify your mobile phone. Successful verification will enable the login approvals
7. Now onwards when you try to log into Facebook from a new or unrecognized computer, you need to enter the security code which one is messaged to your mobile

If you lose or forget your phone and have login approvals turned on, you can always return to a previously recognized computer to regain access to your account