Product templates are used to show product page. Out of the box, in GrandNode, you have two types of products views - product simple or grouped (with variants). If you want to create a new template for your product. You need to create a cshtml file, then add it to your store to the Views/Catalog folder.
After that, you need to go to System -> Templates -> Product templates -> Add new record
In the name field just enter the name of your template, view path is the name of the file which you added.
Please note that you need to restart application when you add new file to the store.