Coupling is using 2 (or more ) functions at the same time. For instance, rudder and elevator or rudder, aileron and elevator.

Computer radios have the ability to do this through the programming. MOST computer radios refer to it as "mixing". Check the manual for your radio to see how it is done for your radio.

I have a couple of planes where I have coupling/mixing activated. One uses elevator with the flaps, another uses rudder and elevator with the ailerons (scale 30's era plane).

Hope this helps.