A favicon is a small graphic file that needs to be inserted in the template root /templates/”your template” and named favicon.ico

The file itself can be created in standard images editors at 16px x 16px or various ico generator websites, including:

converticon.com/
www.favicongenerator.com/
www.htmlkit.com/services/favicon/
tools.dynamicdrive.com/favicon/
www.favicon.cc/

With the favicon.ico file in place it will be displayed in browser tabs, browser history and bookmarks.

For more information please check out these links

docs.joomla.org/Changing_the_site_favicon
en.wikipedia.org/wiki/Favicon