Favicons are used to improve user experience and tab-navigation in a web browser. "Favicon" combines the words "favorite" and "icon", and was first introduced by Microsoft Internet Explorer 5 in 1999. They're that little 16x16 pixel icon thats appears next to the site name in tabbed browsing and in bookmarks.
How do I add one to my NationBuilder site?
Go to Website > Site settings > Favicon to upload your new image.
Enjoy your new Favicon!