Favicon (short for favorite icon) is the icon shown on browser's address bar or next to the site name in the bookmark list. It will be displayed on address bar when your affiliates sign up or log into their account.
Favicon helps your affiliate program to be more professional. To know how to Add a favicon to your Shopify store, you can follow this guide from Shopify.
To get your Favicon URL, go to your webpage and right-click > View page source.
Press Ctrl + F > type shortcut icon > you will see your favicon URL next to it.
Copy the URL and paste it into the Favicon URL box:
Finally, don't forget to click Save.