What is Preferred domain setting?This is the way to tell Google which URL you prefer would like Google to index. like http://easkme.com or http://www.easkme.com. If you don`t do this then chances are Google will penalize you for copy content.
Lets see what Google talk about preferred domain :
|Set Preferred Domain Setting Using Google Webmaster Tool : eAskme|
Check more here on Google official page.
How to set up preferred domain using Google webmaster tool?You just need to verify domain ownership of domain with both www and without www. It verify same meta verification to verify ownership, when you submit your website on Google webmaster tool. Login to your website dashboard and click on gear icon then click on site settings.
Now you can set your preferred domain. Add both www and non www URL of domain.
If you have already set proper 301 redirection then you can skip this step.