Data Import makes mapping selections for you automatically when it picks up on and remembers selections from prior imports. Automatic mapping can save you steps in the import setup process.
You can change mapping selections using controls in the top two rows of the Data Mapping preview. If you are using a template for your data import, you cannot change mappings unless you detach the template (click the “x” button to the right of the template name in the mapping window).
See also Automatic Data Mapping And Import Configuration.