"Span" refers to the part of the Earth you are currently looking at on a map page. It is possible to center in on one area and search for Places only within that area. Here is how to do it.
1. Zoom in and center your map on the area you are interested in searching.
2. Enter the word or words you want to search for in the "what" box of the blue search bar, but do not plug anything into the "where" box.
3. Now you will see all of the Places matching your search appear in the map.
You can move the map around, zoom out, zoom in, and the search results will change as you do so, to correspond to the current span.