The iCloud Lock (Or Apple ID lock) is a problem that many people have, especially the ones that purchase second hand devices.
An iCloud Locked iPhone, will basically forbid you using an iPhone, unless you successfully enter the correct Apple ID (e-mail) and password of the iCloud account that this iDevice was activated with.
In general, the iCloud Lock security feature Apple has added, is a good thing. It prevents Stolen or Lost iPhones, to be resold, and it makes those iDevices unusable.
iCloud Lock makes your iPhone, iPad or Apple Watch not usable.
This post will help everyone to remove or bypass the iCloud Activation Lock from your their iPhones and make them usable again.