It connects the hydraulic system for your rear brakes to the e-brake so you can put a lot more pressure on ONLY the rear brakes and not the front. You're also going to need brakes aimed at stopping (which you probably already have).
It's not a terribly difficult modification and definitely not an expensive one. One thing to note though, you have no wire e-brake after the conversion, just the hydraulic one. So if you lose brakes, you've lost any hope of stopping. If your car is manual it isn't an issue because you can just use the gears to slow down. The pedal will still apply front and rear brakes.