Whenever someone fills in your published email form a small window appears indicating that the data has been sent.
The email form owner (you) can add his own short message to this window (to say thank you, for instance).
Steps to add a thank you message:
1) Go to the “Thank You Message” textbox in the “form settings” box on the right.
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.