An older style of camera design which has a focussing system based on mirrors and triangulation. It's surprisingly fast and effective, and is still used on some modern-day Leica cameras.
You view the scene through a separate 'direct vision' viewfinder on top of the camera, but there's a second viewing window a little distance away. As you turn the focussing ring on the lens, it rotates a mirror inside this second window, and when the main viewfinder image and this secondary image line up, the subject is in focus.
It's fast and effective, though it does require precision engineering and top-quality components. It was also used in Epson's R-D1 rangefinder, now discontinued.
