Data preview mode - indicated by on the toolbar - is a way to preview your data model before you create or save your API. You can view the structure in JSON, CSV and RSS and you can also download your results directly from the toolbar (thus bypassing the API creation stage and completing a quick one time pull -- especially helpful for sites that make it hard to kimonoify!).
If you like what you see, go ahead and finish creating/updating the API. Otherwise you can edit your API by navigating back to the extractor view to add or remove datatypes, or navigate to the data model view to modify the CSS selectors, attributes extracted and adjust the data model.