If you ever need to undo your import and start over, you can choose to revert it any time within 48 hours of its initial import with the Revert button.
Once the data import has been successfully finished, the Undo button will be shown. In any case you would like to undo the data import and remove all imported records, you could do so by clicking on this button. It will only remove all records that are associated with the particular import file. This way, if you made a mistake on the field mapping and you want to re-do the import, you could do so.