The overview map helps you keep perspective where on the Earth you are looking at, and is a fast way to go to another part of the world you want to see. You can turn the overview map on and off by keyboard shortcut: Control-M (a different combination on the Mac). You can also adjust the size of the Overview Map by going to the GE Options (under “Tools->Options” on Windows, or “Preferences” on the Mac). Here you can also change the “Zoom Relation” which means how close the Overview Map matches the zoom level of your current view (default is infinity, a smaller value has the zoom level more closely match your view). The smaller-value zoom relation can be handy if you need to compare your view to a slightly larger zoom level.
Related: Google Earth User Guide on Overview Map
About Frank Taylor
Frank Taylor started the Google Earth Blog in July, 2005 shortly after Google Earth was first released. He has worked with 3D computer graphics and VR for many years and was very impressed with this exciting product. Frank completed a 5.5 year circumnavigation of the earth by sailboat in June 2015 which you can read about at Tahina Expedition, and is a licensed pilot, backpacker, diver, and photographer.