It you mean those lines that maps have with a distance measurement written above each, then it's like this: The line shows the scale of the map. If it says 1 km, and the line is 5 cm long, then every 5 cm on the map is 1 km in reality. If you mean the ruler in the image.... I see "1km" written on it, so it's probably the same as those lines on the map, just have to make sure you use the right ruler for the right map. As for how to use it specifically.... There being a different measurement type at a right angle probably means something but I dunno. I refer you to Google-sensei.