A “ducker” is an audio tool (traditionally hardware, but now also in software) that reduces the level of one track when there’s a signal on another track. It’s very common in talk shows, where you want the host of the show to have priority and override the voices of guests. A ducker is extremely useful for cleaning up two-track interview recordings, such as those from Skype.

As of release 1.3.3 (currently in beta), the open-source audio package Audacity has included an “Auto Duck” feature, which could be quite helpful if you record/edit two-track interviews, particularly from phone calls or Skype.