This option lets you generate files that Form Publisher failed to create upon form submission, or regenerate files in order to have them match a new template of your choice.
If files have not been generated, you can still retrieve them by using the 'Regenerate files' option in the add-on menu:
Once the sidebar appears, pick the day when your files have not been generated, and select the different form responses you would like to use in order to regenerate files.
If you keep having difficulties generating files, it might be because you have reached your monthly quota. To learn more about it, read the following documentation: Form Publisher files quota