Hid kits

[oakley990099]

hi i am looking to purchase hid kit off of this site http://www.xenonhidworlds.com/

and i was wondering when they ask if i would like to purchase an optional "relay harness" , what exactly that means? and do i need this in a 03mazda6 s??

also is this everything i would need to convert my headlights? http://www.xenonhidworlds.com/HID-Headlight-Kit_22287.html

thank you