What is the purpose of license key for Licensing & In-app billing

Can someone please explain me, what is the purpose of license key for in-app billing ?
 

I adjuested my application- in-app system according to the tutorial
/http://www.coronalabs.com/blog/2012/03/13/getting-started-with-android-in-app-billing/

and Corona Sdk example. And I just provided productId while using store.purchase( {productId} ) method.

Do I need to make some changes according to new Google play in-app system or do I need to do anything with license key on google play account.

Thanks in Advance