a coupled rangefinder means the rangefinder -- the optics that measure the distance -- are built into the camera and linked to the lens, so as you focus the lens you match the patches in the rangefinder. The rangefinder window on screw mount leicas is separate from the viewing window. On M-series leicas it is built into the viewing window.
Originally Posted by StoneNYC
an UNcoupled rangefinder is a rangefinder that is separate from the camera. You us it to find the distance, set that distance on the lens and take the picture.
A parallax corrected viewfinder is one in which the frame lines in the viewfinder adjust for whether the object being shot is closer or farther away. Generally, higher quality rangefinder cameras with coupled rangefinders that are build into the viewing window, such as M- leicas, have these. I think the IIIg does too, but don't quote me.