Topic templates are used to show topic page. Out of the box, in GrandNode, you have one type of topic view. If you want to create a new template for your topics. You need to create a cshtml file, then add it to your store to the YourTheme/Views/Topic folder.
After that, you need to go to System -> Templates -> Topic 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.