A diplexer is used for combining (and separating) satellite and terrestrial (antenna) signals.  Typically you would use one diplexer at the satellite dish and antenna on the roof to combine the signals together.  These combined signals travel down a single coaxial cable to the wall socket where a second diplexer is used to separate the signals and voltage to 2 separate cables.  One for satellite and the other for the antenna.  Very useful when it is near impossible to run a second cable for the satellite dish.  Simply share the antenna cable for both.