Acoustic coupling refers to acoustic impedance: the degree to which sound is transmitted across some acoustic barrier.
This can be exemplified by keeping your mouth and nose closed and producing a voiced sound. No air escapes, but you can hear a sound because the sound waves produced at the glottis can vibrate through the cheeks. In fricatives, the front and back cavities are not well coupled, and there is a large acoustic impedance at the constriction. Although air flows through the constriction, relatively little sound is transmitted to or from the back cavity. This means that the resonant frequencies of the back cavity have relatively little effect on the spectrum of the fricative.