A Thank You page (also called a Completion page or a Confirmation page) is the page shown just after a conversion has happened.
Thank you pages are used as a confirmation to the the user (customer) that the action has been performed correctly (e.g. purchase made, form submitted, etc.).
To track conversions, you need to add the conversion code snippet inside the <body> </body> of this page. This code will help ClickMeter record the conversions.