Category templates are used to show products in categories. Out of the box, in GrandNode, you have two types of category views - products in grid or products in lines. If you want to create a new template for your categories. 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 -> Category templates -> Add new record
Please note that you need to restart application when you add new file to the store.