What is the use of node--product_display.tpl.php? I am new to Drupal Commerce, what is the use of node--product_display.tpl.php?
node.tpl.php is the default theme for all content types in a drupal installation.  If you want a specific content type to have different layout/theme you can override by creating a tpl.php file like this node--[CONTENTTYPENAME].tpl.php so for example for ARTICLE content type your would copy node.tpl.php and rename to node--article.tpl.php and adjust the new file to as you desire.  

Now Commerce displays a product using a CONTENT type called product display thus to change the look of this content type you will create node--product_dispaly.tpl.php

here is some more explanation of things that you can do inside a node.tpl.php file and (all overrides) https://api.drupal.org/api/drupal/modules%21node%21node.tpl.php/7.x