Nowadays Smartphones are the most vulnerable devices for theft because of their very high prices. Now premium smartphones like iPhone and Samsung Galaxy devices (e.g. Galaxy Note 5, Galaxy S5 / S6 etc.) costs to Rs. 30000/- to Rs. 80000/- that made them hot object for thieves. Thieves stolen them and sell to the grey market at very low lost (less than 25% of the mobile cost). Now these grey market people sell them to users at 50% of original price via mobile resell shops, websites like olx.com etc.
Many people those are getting the premium phones at fifty percent of price or even lesser doesn’t care about their bill and source. That’s how business of stolen mobiles works. Samsung has come with reactivation lock to secure your smartphones from unauthorized access and from theft. (Mobile buying guide)
What is Reactivation lock?
Samsung reactivation lock requires your Samsung account username and password to unlock your smartphone in case you factory reset your phone. After factory reset completion when phone boot up it required to input your Samsung account username and password to unlock otherwise one who have stolen your device or found a lost device would not be able to use the phone. This way nobody can use the device with reactivation lock activated, even they can not flash custom ROM, flashing a custom recovery or any other custom firmware using Odin.
How to enable reactivation lock-
Go-to settings-> security, in reactivation lock check to enable. Now you will be asked to enter Samsung account password if already activated Samsung account, otherwise you need to register with Samsung to create your new account under Account section in settings.
At the time of you handed over the phone you to service center you required to unlock the device.
Some webpages says that this can also be cracked (on some XDA forums etc.) but it is advisable to enable this security feature for extra layer of security in case of lost / stolen your device.