Activation Lock is a security feature on Apple devices that runs on iOS 7 or later versions. Using this feature you can prevent the reactivation of lost or stolen devices.
Activation Lock is enabled automatically when you turn on the Find My Device feature on a device. To enable Find My Device, log into iCloud account on the device. If you sign out of the iCloud account on your device, Find My Device and Activation Lock will be disabled.
If Find My Device is enabled on a device, iCloud credentials are required to:
Erase the device
Reactivate the device
Turn off the Find My Device feature
If Activation Lock is enabled on a corporate-owned fully managed iOS supervised device, iCloud credentials are required to reset the device and assign it to another user. If iCloud credentials of the user are not available, then administrators can use the Activation Lock Bypass feature to bypass the Activation Lock on the device.