You can get the details here. It cost about $299.99
There is also another popular and cheaper alternative from MTX called Re-Q5 for about $179.99
I can't say I have experience with the MTX but I've installed a few of the JL cleansweep. There are others brands but these two are the most popular.
Basically in a nutshell this is what it is.
EVERY stock deck has a curve in the processor that is designed for the stock speakers. When replace the stock speaker, which has a different curve, you are forcing the aftermarket speaker to play in a way that it is not designed to. That is why it may sound flat, tinny or not as good.
When you introduce the new sound processor(JL cleansweep or the MTX version), it takes the signal from the deck, clean it up and sends a clean flat signal to your aftermarket speaker or an amp.