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A "front end" program is one such as MediaCar, MediaEngine, CarBox, FrodoPlayer etc, which sit "on top" of your Operating System (in most cases Windows) and give you a graphical interface to the computer for playing music, videos and in many cases also GPS.
Most of these "front end" programs are skinnable, which means that people can create new "skins" for them, to customise/change the look/feel of the program.
There is a LOT of information here in the forum, just remember the is your friend