Content files in Kirby don't have a predefined structure. Actually you can use any set of data for each page, which makes the entire system so flexible. As soon as you are using the Kirby Panel, you somehow need to define the form fields for each template.
Those form field definition files are called Blueprints.

All your blueprints for your site are located in site/panel/blueprints. After installing the panel, you will find the blueprint for the default template in there, which you can use as a starting point.

You must define a blueprint for each of your templates, otherwise the default blueprint will be used.

Blueprint Definition

Blueprint definition is very easy and it only takes a few minutes to set up the needed form fields for each template.

Kirby uses YAML to define form fields. It's a simple, intuitive language to define data structures. But I guess some examples make it clearer.

The Default Blueprint

The default template, which is shipped with Kirby uses only two fields of data. title and text so the blueprint for this template is very straight forward:

title: Page
pages: true
files: true
    label: Title
    type:  text
    label: Text
    type:  textarea
    size:  large

As you can see YAML uses indentation to create a basic structure. The rest of the language looks pretty much like the definition in Kirby content files.