A CCTV multiplexer or Closed Circuit TeleVision Multiplexer allows more than on camera to be connected to the unit but only one output at a time is allowed through the unit.  Most Digital Video Recorder or DVR units have this function built right in to the DVR.  Another way to describe a multiplexer is by its function which is to combine various input signals and generate only one output signal.  You can almost think of a CCTV Multiplexer as being similar to a computer router.