iCloud: Activation Lock
With iOS 7 or later, Find My iPhone includes a 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
Check a device’s Activation Lock status
If you’re buying or selling a used device, you can check if the device is locked at icloud.com/activationlock from a Mac or PC.
Erase a device before you sell it or give it away
Be sure to erase your content and settings (in Settings > General > Reset > Erase All Content and Settings). When you erase your content, Find My iPhone and Activation Lock are also turned off. For more information, see the Apple Support article What to do before selling or giving away your iPhone, iPad, or iPod touch.
If you no longer have the device, follow the instructions to remove a device you no longer have.