It won't fit on your 04 without a serious lift.
Offset is where the backside of the mounting pad is located on the wheel from center. Back spacing is the measurement from the back of the mounting pad to the back side of the wheel. Typically speaking, the higher the offset, the more the wheel will tuck inwards towards the suspension. The lower the offset, the more the wheel will push out towards the fender.
If you have a -12mm offset on a 9" wide wheel, the mounting pad would be 12mm towards the back
of the wheel from the center. If you have a +12mm offset on a 9" wide wheel, the mounting pad will be 12mm forward