The term conversion usually describes an action completed by a prospect on your website, such as a registration, a subscription, a purchase or downloading a specific item.
Prospect.io allows you to track those actions so you can measure the results of your campaigns.
In order to do that, you need to copy the code that you can find in Settings > Mails > Tracking.
Place this code on a "Registration completed" or "Thank you for subscribing" page. Anything that best describes a conversion for you. It just needs to be an HTML page online.