The A vs B Testing feature allows you to create a copy of a popup that you can change the styling and content of to see if it converts more. This is done by either showing Popup A or Popup B to a customer at random.
For example, if a user views your site they have a 50% chance of seeing Popup A and a 50% chance of seeing Popup B. This gives you good testing data to determine which popup is better for your site.
Please let me know if you have any other questions :)