Basically, you invoke a keyboard command to bring up the Site Launcher overlay (which can be as transparent as you want), then press a key to load a particular site. The extension comes with various popular sites built in, but obviously you can replace them with your own.
You can see Site Launcher in action in the above video. In addition to the obvious benefits of speedier access to your favorite sites, Site Launcher gives you the option of removing the Firefox toolbar, thus gaining some extra screen estate -- something that could prove beneficial to, say, netbook users. [via Webware]