Basically Googlebot needs a lot of resources like image files, Java script and CSS to render pages for search. Webmaster also uses such resources to make them active and better operative. So, what Googlebot will do, if these resources are blocked? That is reason that this new feature has been added to webmaster that you can unblock such resources using your own website.
Using Block resources report you can examine the names of the hosts from which your site is gaining block resources. By clicking on the row as the image mentioned above, you will be able to find the respective pages as well as guide to resolve the matter that the resources should be crawled again by Googlebot.
Fetch and Render report will help you understand how this blockings matter. Through this it will be clear that Googlebot determines such pages differently for crawling. Webmaster tool will show you the sites that have influenced you and you should up-date the robot.txt files for the same respective pages. If you want to operate the same for all the pages it will take a longer time. You may take a visit of help and support centre or webmaster help forums for a better convenience.
It can be anticipated that the new feature will help the website owners and hosts to find and unblock such resources in an easier way.