It basically draws power directly from the battery instead of trying to run the higher demand through the stock wiring. Sometimes you're ok but sometimes thing melt or fuses blow. Also you can run into minor issues like flickering if the stock system can't handle the request for enough power from the ballasts. The harness just protects and powers everything nicely. It is really really easy to install with the HID kid. It looks complicated but once you lay everything out it's not bad at all.
If it looks scary when you get it, just lay out each component and unfurl the harness. Just match up all the ends, the connectors should make it pretty obvious what goes where and I bet it will make more sense to you. You will have to attach the red (usually) positive end to the positive battery terminal (also usually red) and find grounds for the black (usually) ends of the harness. Most of the time there are already ring connectors attached so just find a screw somewhere near the ballast on each end and stick the ground to that point. Might need to file the paint away a little to ground it well.
If you're feeling overwhelmed let us know and we can try and help. If you feel like you don't want to tackle it, bring your kit to the May Dragon and I can help you install it in like an hour. Or drive all the way to Louisville, which is probably unreasonable but offer is there. Hahaha.
Here is a great simple diagram of where everything goes:
1) Red Power wire with Fuse to the positive on the Battery, the fuse is included with the harness.
2) Long red wires to each ballast
3) Each ballast plugs into each bulb
4) Black ground on the end next to each ballast with the "ring" connector
5) The black box at the top is the relay, the cord on this plugs into the stock system so your headlight controls work just like always.