Offset refers to the position of the hub relative to the width of the wheel. 0 offset means that the hub is exactly in the middle of the wheel, so there is equal width on either side of it.
+48 is about as high an offset as our cars can fit, and honestly idk what Subaru/Toyota was thinking putting them on our cars. The ideal offset varies with the size of the wheel you choose, so for 17x8 the best choice would be +36 to +38.
Edit: Here's the link for the offset chart.
http://www.ft86club.com/forums/showp...40&postcount=2