The thank you message will be what the user sees directly after they submit the event form.  A copy of it will be emailed to the user as well. It is there to alert the user of the transaction they just submitted.

Click the blue Edit Message button on the bottom right to make changes. The email will open in the email template editor. Double click in a text field to edit the text and add Mail Merge fields that can display information about the registrant as well as their registration. Details about which Event Levels and Add-Ons were selected and the breakdown of the cost, fees and even any money is owed are included in the template. 

If you have provided a Pay Later option, a second letter will also be available for editing to include information about where to mail checks and when payment is due.