Basically, every time you (or someone else) try to access your Facebook account, whether it be through the internet or the application, the program will display a lock screen and you will have to put in your four-digit code in order to gain access.
As a result, with Facebook Secure you can rest assured that no one else who takes your iPhone will be able to read your private Facebook conversations. This is a relief for everyone who values their privacy.
Facebook Secure adds an extra layer of protection to your cellphone. If you tend to let other people use your phone, this program is for you.