It is a small graphic that can be made to appear in the address box on the browser to the left of the URL. On most sites it is the ubiquitous blue E of internet explorer.
If you create one, you call it favicon.ico and put it in the root directory of your website. When you book mark a site with a favicon, and sometime go back to your bookmarks, the favicon should standout from among the blue Es of the other bookmarks.
Well in theory this should happen