After selecting the import button Transaction Pro Importer will validate the data in your import file for required fields, proper data type etc. If there are any errors in your data you will see a validation log where any data that is missing or is incorrect is highlighted in yellow. You need to hit the back button and either modify your import file or your mapping and then retry the import.
Once all the data is correct and the data was imported an import log will be displayed on the screen with a status column. If any of the rows have a status of error these rows did not import and you will see the reason why they did not import in the message column.
To verify which transactions were imported you can review this article