You need software to perform whatever functions you want your PC to perform. If you want GPS navigational capabilities, you need GPS navigation software to perform that function, as an example.
A frontend is a program that gives you easy access to different functions of your vehicle computer (music and/or video selection, GPS navigation, web browser, OBDII functions, etc.). FrodoPlayer is one frontend.
The frontends that are written by the forum members are "skinnable", meaning you can make it look the way you want. You need a skin when you get sick of the way it looks.
I suggest you peruse the FAQ Emporium and do some reading.