Nowadays, most new cars offer at least one way of connecting to an external playback device, via Bluetooth or wired auxiliary input or both. A decade ago, though, this was more of a premium feature. Back then, if you wanted to connect your music player or smart phone to your basic stereo, the simplest and least invasive way was a device that plugged into the headphone jack of your device (or alternatively connected to it via Bluetooth) and which broadcast a very low power FM signal. You would the tune your car stereo to the matching frequency, and your device's audio would be piped through the car's speakers. Usually the transmitter provided a way to choose from a list of frequencies so you could pick one not used by any local radio stations. When operating in good conditions, if another car was driving down the road next to you, they could also tune to the same frequency and hear your playback.

I quite often have to sit next to people watching YouTube videos, or football games with through their speakers; why spend money on ear phones?