a front end is a program that is used to easily control another program(s) ... or to provide services created from another source.
Roadrunner is a frontend because it provides an easy way to control and play your music / videos / GPS etc ... with the music / video being played by Winamp.
A frontend is an "interface" that allows a user easily control or manipulate another piece of software or hardware... Windows XP is a front-end that allows you to control and manuiplate the services offered by the computer hardware.
In terms of Roadrunner (RR), you can still manually do all of the things RR does without having RR on your computer, but it takes a lot of time and effort. Installing RR just simplifies the process of playing music and video etc and put all of the necessary controls in one easy to use interface or "frontend"
Hope this helps ...