While you design your graph, it's sometimes useful to see how the graph might look in the CMS or the APIs when published. That's where preview comes into play. It allows you to preview your changes before publishing.
It can be accessed from the top right by click on the 'Eye' icon on the Graph page. When you click on it, a modal opens up with the preview.
You can switch between 2 types of previews:
Allows you to see how the graph translates to the CMS. You can interact with the different input types and even trigger the validations to ensure that your graph translates to the CMS just as you intended.
See how your graph translates to the API. You can see a sample response mocked with sample data for the fields that you defined. The mocked data however, realistically represents the type of data returned. This can give you a good idea of how your table's fields would be available for consumption in the API.