The Page Template is the default code that makes up your exported page.
When you click on it it will display a text area that you can use to modify the page template.
You can change this but doing so may break your page.
But don't worry, the Default button will reset it.
You can also edit the page template by opening the Element Options panel, selecting the artboard and then clicking on the template button.
If the page template has been modified the icon color is changed.
The template code has a list of tokens that you can use to determine what is inserted into the page and where.