This is an older camera designed still used by celebrated German manufacturer Leica. The ‘rangefinder’ is used for manual focusing – as you turn the focus ring on the lens, a small mirror in the top of the camera rotates to line up a ‘ghost’ image with the main image in the viewfinder. When this ghost image lines up, your subject is in focus.
This uses the optical principle of ‘triangulation’, which has a similarity to today’s phase detect autofocus systems. With phase detect AF, the camera compares two versions of an object or detail ‘seen’ from two slightly different positions, and can determine how much the focus needs to be shifted and in which direction to bring the object into focus.
Getting back to rangefinder cameras, these take a certain amount of dexterity and skill to use, but those who have mastered their operation become lifelong fans. You can swap lenses on Leica M rangefinder cameras, which technically makes them interchangeable lens cameras alongside DSLRs and mirrorless cameras. Indeed, Leica rangefinders are themselves mirrorless cameras, but predate the modern versions by many decades.