What is activation lock

Activation Lock is a feature available on iOS 7 and later and increases the overall security of the device. If, in any case, your device is stolen or locked, you can remotely restore/erase the device using Find My iPhone. Or, even if the thief gets hold of your device and restores it themselves, they still would not be able to unlock or activate it because they need to log in with your Apple ID and password.

Learn more: https://support.apple.com/en-us/HT201365

it's a theft protection

you can read more Activation Lock for iPhone, iPad, and iPod touch - Apple Support