A guy that i get all of my car audio stuff said that it would be a good idea to buy a line driver. But what the hell does it do... What is its purpose?

 

A line driver boosts the signal from your HU to your amp. This may or may not benefit your set-up. If your HU is providing your amplifier with enough voltage to power your speakers at your desired ilstening level without audible or damaging levels of distortion then you probably don't need one. I have noticed that HU's rated as high as five volt pre-outs don't even put out 2 volts near max power (say 90% power), at which point there is a high risk of damaging levels of distortion in the signal. Adding a line driver will boost lower, undistorted voltage signals from the HU to higher, still undistorted levels at a higher voltage. A main benefit here is that you can ususally ease off of the gain on the amplifier to allow the amp to provide the rated RMS and still sound great.

Note - when I use the word undistorted I really mean low distortion (not auditble and not damaging to speakers).