Billy pretty much got it right with retrofit. It's putting something new into something old that didn't originally have it.
P&P = plug & play, which means all you do is literally plug it in and go. There is no modification necessary.
Please do not get a P&P kit for your stock halogen reflector headlights. If you're going to get an HID kit, retrofit projectors into your headlights. From my past research, eBay projectors are garbage. Your best way to ensure you get the best light output and cut off is to retrofit projectors into your stock headlights.
There is a sticky I recently created regarding P&P vs. OEM / retrofit. Do read
. It explains why you shouldn't use a P&P kit with stock halogen reflectors.