"favicons" are small icons that most browsers display to help users identify websites.
! favicons in the Browser
When TiddlyWiki starts up in the browser it looks for a tiddler called [[$:/favicon.ico]] and dynamically uses it as the favicon for the page. If you modify the image then the favicon changes instantly to reflect it.
! favicons on the Server
On the server, the ServerCommand will serve the tiddler [[$:/favicon.ico]] at the path `/favicon.ico`.