The background map provides a geographic reference for the topology. The default map is the Continental US. The map layer's visibility can be toggled on and off with the 'B' (for background) key.
See Providing Alternate Maps for the Topology View for additional details.
The scale of background map can be changed on demand. To do that, simply append ?mapscale=<real-number> at the end of the URL. For example,
The Topology Sprite Layer mechanism allows custom "background" elements to be displayed (above the background map, but below the topology elements). This could be used, for example, to inject labeled "subnet clouds", or the plan of an office building or campus into the topology view. Typically, one would want to hide the background map layer (press 'B') and show the sprite layer (press 'S') instead.