The offset is the distance the inner hub surface of the wheel is from the imaginary centre line of the wheel. So if the inner face of the wheel that makes contact with the hub is on the centre line the offset is zero. The distance the inner wheel hub surface is from this centre line will determine a wheel's offset eg ET38 - hub face 38mm from the centre line of the wheel. Offsets can be positive or negative. It is important to get the right one for your car. The Bravo HLX alloys are ET38.
With a larger or wider wheel the centre line of the wheel will change which will mean the offset will need to change accordingly to accommodate the wheel so that is doesn't foul any part of the bodywork or suspension under any condition.
Hope this clears up any misunderstandings.
If you are still confused vist this link which explains all you need to know about tyre and wheel dimensions.
