the offset is how far from the centre of the rim the mounting point is. So if you looked at a terrano/mevrick wheel the bit that touches the hub would be near enough central to the whole rim.
Minus offset is towards the middle of the car, positive offset is towards the outside.
Nothing fits straight on, so you'll need spacers as well. Frontera rims need a 10mm spacer. You won't need hubcentric spacers though as 6 bolt wheels centre themselves on the bolts. So, as long the hole in the middle is at least as big as a terrano hole, and the pitch circle diameter is 139.7, You're larfin, :lol