Hello there,
To "couple" usually refers to using a capacitor to inject an AC signal into a circut while eliminating the DC component of the signal.
To "decouple" is the contrary, to reduce the AC signal while maintaining the DC component.
Decoupling is usually used to get rid of noise, which is an AC signal of varying frequencies and/or amplitudes, or to reduce the effects of a signal in one part of the circuit from interfering with another part of the circuit.