With iOS 7 or later, Find My iPhone includes a new feature called Activation Lock, which is turned on automatically when you set up Find My iPhone. Activation Lock makes it harder for anyone to use or sell your iPhone, iPad, or iPod touch if it’s ever lost or stolen.
With Activation Lock, your Apple ID and password are required before anyone can:
Turn off Find My iPhone on your device
Sign out of iCloud on your device
Erase and reactivate your device
Important: Make sure to remember your Apple ID and password, and that your password is unique and secure—someone else shouldn’t be able to guess it. For more information, see the Apple Support article Security and your Apple ID.
Before giving away or selling your device, be sure to erase your content and settings (go to Settings > General > Reset). When you erase your content, Find My iPhone and Activation Lock are also turned off. If you no longer have the device, follow the instructions to remove a device you no longer have. For more information, see the Apple Support article What to do before selling or giving away your iPhone, iPad, or iPod touch.
For more information about Activation Lock, see the Apple Support article Find My iPhone Activation Lock in iOS 7.