Ducking is using a compressor to turn down 'Signal A' whenever 'Signal B' starts.
For example, a dj is playing music and he wants it to get softer every time he talks into his microphone so that he can be heard clearly. He could set up a compressor so that the signal from his mic is split to the compressor 'key input' while the music passes through the compressor itself, that way whenever he says anything into the mic the compressor will kick in and 'Duck' the music.