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.

Available on the thank you message are several Mail Merge fields that can display information about the registrant as well as their registration. Including details about which Event Levels and Add-Ons were selected and the breakdown of the cost, fees and even any money is owed.