Your Retargeting Campaign might have a specific goal (signup, sale, form submission, etc.).
Once that goal has been achieved (on a per-user basis) you no longer want to waste your budget advertising to those users.
If you are selling a product, for example, you no longer want to advertise to users that have purchased it.
To accomplish this, you need to create a second audience tag that will be placed on the Conversion Page, or Thank You page of your website.
Once the second tag is in place, you’ll create a Retargeting Campaign which targets your main audience and exlude the converted audience.