A coupling capacitor is a capacitor which is connected in series to a circuit. The function of the coupling capacitor is to block DC and permit only AC to pass through. Typical examples would be an audio circuit which requires only the amplified speech signal which is DC to be passed through.
The impedance offered by the capacitor is inversely proportional to the frequency. Thus, the capacitor offers low impedance to the AC signal and high impedance to the DC signal.