If you want a general-purpose post about device security or how Activation Lock works (legal, educational), tell me the tone and audience and I’ll draft one.
The iCloud lock is tied to the device's Unique Identifier (UDID) and the user's Apple ID. When a user enables Find My iPhone, the device's UDID is linked to the Apple ID, and the device becomes locked to that account. To unlock the device, the user must enter the correct Apple ID and password. However, if the user forgets their credentials or the device is second-hand, the iCloud lock can become a significant obstacle.