iCloud Lock (Activation Lock) is an Apple security feature built into all iPhone, iPad, Apple Watch, and Mac devices. It activates automatically when Find My iPhone is turned on, linking the device to the owner's Apple ID.
When iCloud Lock is active, the device cannot be activated without entering the password of the linked Apple ID. This protects the owner in case of theft or loss, but creates a serious problem for second-hand buyers: if the seller didn't unlink the device from their Apple ID, the new owner gets a locked device that cannot be used.
That's why checking iCloud Lock status is a mandatory step before any purchase of a used Apple device. iSpy lets you instantly check Find My iPhone status (FMI ON/OFF) by IMEI or serial number.