What does activation lock mean

Activation Lock is a form of theft protection designed to prevent people from taking, wiping, and restoring your iPhone or iPad without your express permission. It does this by using your Apple ID. Before an iPhone can be activated with a new account, it needs to be removed from yours.

For complete information on Activation Lock see this article Activation Lock for iPhone, iPad, and iPod touch - Apple Support

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Info...Activation Lock for iPhone, iPad, and iPod touch - Apple Support