You’ve got two security settings on your NOOK Tablet:
Device lock passcode. If you’re worried that unauthorized people may get their hands on your NOOK Tablet, tap here and from the screen that appears, type a four-digit passcode. From now on, whenever your screen locks, entering this code is the only way to unlock it. So make sure you remember the passcode.
Restrictions. If you’re worried about your children or anyone else using your NOOK in a way that might concern you, you can turn off the browser and the NOOK’s social features, such as Facebook. Tap here and type a passcode; a screen appears that lets you turn off those features. Uncheck this box if you don’t want the restrictions in place anymore.