Tracking ID is an alphanumeric identifier that helps you to track your traffic. For example, ClickBank Tracking ID can help you determine which affiliate campaign works better. When you use the same ClickBank feed on two different websites and want to know which one brings more conversions, you can use different tracking IDs for these websites. You can generate two similar feeds with two different tracking IDs ("website1" and "website2").
Then in your ClickBank account, you will see which tracking ID brings more results.