A FavIcon [short for favorite icon] is the small image that shows up next to the name of a site in the tab or address bar of a browser. Favicons are created using editing software like GiMP or Photoshop, and then saved as favicon.ico. The minimum image size should be 16px by 16px. More information on FavIcons can be found here.
To add or change a favicon for a public site in your a2z system, go to Event>Features>Content Management>File Management and upload favicon.ico into Public/Custom Folder.