Offset is how far the hub of the wheel is from the centre of the wheel. So basically the lower the offset, the more dish you will get. Or more spoke concavity depending on the style of the rim. But this also means that the wheel in general will sit further out the lower you go with offset.
In general lower offset = tough, but more effort involved in getting the wheel to sit right if you go too low (guard mods, camber, stretched tyres).
For your car id go something like 18x8.5 front and 18x9.5 rear. With maybe 35-ish offset for something reasonable.