In General
A coupler "couples" or copies a combination of stops from its division (such as the Swell) or pitch range of origin, to another location (such as the Great or the Swell an octave higher). They make it possible for the stop combination on one division--
On an organ without couplers, each manual can sound only the stops that originate with that division. The Great cannot make use of Swell stops, and the Pedal cannot make use of the Swell or Great stops.
There are three main types of couplers: