This is a message in which you can thank the respondents and/or give other information that you want them to receive once they have completed the questionnaire (e.g. when and where the results will be published). You can edit it under the Style tab in the main questionnaire settings. You can include markdown language and can test your message by clicking the button below the text box.
You can also choose not to show a thank you message by switching off the Show 'Thank you' note setting.