If you receive a new SMS message on your iPhone 6, then it will be displayed as a preview on the lock screen. So you can already read what the content of the message is without to unlock the iPhone 6.
This is useful, but each person can read what the content of the message is without needing to unlock the phone. So if you have this message preview on the lock screen of the iPhone 6 and do not want to use it, then do the following steps to disable it on your iPhone 6:
From the Home screen, open the settings, and then:
Notifications --> Messages
Scroll down a bit until you see the section "Message Options". The option "Show Previews" can be found there. Now disable the option by the controller on your iPhone 6.
From now on you will not see the content of a new message on the lock screen of your iPhone 6.