Offset is the relation between the centre line of the wheel and the point the wheel bolts to the hub. A positive offset will pull the centre line closer to the car so you may have trouble with the wheel contacting the suspension or tyres rubbing the inner arch. Negative offset will push the wheel away from the hub which may stick out of the arch or rub the wing.
There is usually a tolerance for each car and I'm sure I've seen it posted online for subarus. It gives you the highest/lowest offsets the car can take.