If you want to show pop ups to visitors directly coming from search engines only check this to Yes.

Scenario:

User A found your page on google and lands on your page with pop ups enabled. If the user decides to go back the pop up is triggered.

User B got on your site through a different page (from typing in your url or from a different page (without pop up) found in google) clicks on a page with pops enabled. In this case the ad is not being triggered because the user is not SERP direct referral.