The Ruler is a tool to show the straight line distance between your location and any point you select on the map.
To use the ruler, press the lower button when on the map view (during exercise or without exercising). When the Navigation options opens, scroll down and toggle on Ruler. Go back to the map view and select any point on the map to measure its straight line distance from your current position. When you don't need the ruler anymore, you can turn it off in the Navigation options.