Display Condition: Show to New or Returning
If you want your popup to only be showed to visitors who are new to your site or returning then this is the option for you.
When you edit your popup, you will want to go to Display Conditions.
In there, scroll down to Show to New or Returning, then turn that on. In there you can select New Visitors or Returning Visitors.
After you are done, you can close the Edit Display Conditions (top-right corner) then save / publish your popup.