If you have never used a rangerfinder camera before, you may not know how to focus using one. Here is a short video showing what is looks like when you look through the viewfinder of a rangefinder camera.
As the focusing ring is turned, the image in the rangefinder patches moves. When the images align, the object is in focus.
This video shows a Leica M2 in action. In case you are wondering, the framelines (not shown in the video) and the patch move to account for parallax as the focusing distance changes.
If you want to learn more about how rangefinders work, read the rangefinder section at this link.