What is activation?
Activation helps verify that you are using genuine Windows and that Windows has not been used on more computers than the Microsoft Software License Terms allow. In this way, activation helps prevent software counterfeiting. The information collected during activation will not be used to identify or contact you.
It’s required
You have 30 days after installing Windows to activate it online or by telephone. You need to activate Windows before this 30-day period expires. 
Note:
If you choose to automatically activate Windows online, automatic activation will begin trying to activate Windows three days after you log on for the first time.