The 'thanks page' is the page a participant sees once they've completed the plan.
Note: Only the owner participant will see this page. If an invited participant completes the last task, they'll be redirected to the main plan page which will show as completed.
You can update the thanks page by opening a template, and clicking Template settings on the main template page. Then select Thanks page. You'll see a page like this:

You can make changes to the title and the message that the participant sees.
Enabling Also ask for feedback adds a star rating and feedback option to the page too.
Remember to click Save changes when you're done.