When a user has processed a form you may notify him with a feedback. This could be something like:
Thank you for your input, you'll be notified, if your request has been processed.
To do so activate the properties of the Form and click on the Edit-Button of the Feedback property. You may then edit the feedback in the advanced property dialog.
Is is also possible to include information from the workflow in the feedback. Please refer to Referencing data in Notifications for the details.