Great question — answering as the UphillTech team:
The first box is for setting up the landing URL on your site. The URL that is specified (along with all its tracking information) is sent to your site when the user clicks on the ad. The second box is for setting up conversion tracking using the Facebook pixel. It gives you code to put on your "thank you for your purchase" page that sends data back to Facebook letting FB know that the user performed your desired action on your site.
Full answer by Stephen Ostermiller: https://webmasters.stackexchange.com/a/126741 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
