It's a camera that uses parallax to gauge the distance to the subject and help you focus. Usually the rangefinder (that measures distance) is coupled to the focusing ring on the lens, so that once you've found the proper distance you've also focused on the object you're interested in.
In practice this means that you look through a viewfinder lens, and there's a small region in the middle that shows a split image that aligns once you've found the distance (and achieved focus). It's done with mirrors.
Also, unlike an SLR, you do not look through the lens itself. You also must use a different viewfinder / rangefinder for different lenses (or maybe the camera lets you set your lens focal length or measures it somehow).