Whenever a person fills in a published form a small window appears on the form submission indicating that the data has been sent.
A user can add his own short message to this window (for saying thank you for instance).
Steps to add a thank you message:
1)
Go to the "Thank You Message" textbox in the "form settings" box.
2)
Enter your short message and press on the "apply" button.
3)
Once the published form is submitted, a window will appear containing your message.