That's 17" diameter, 8" from front lip to back lip. If you look at the rim from the front, so that it's rolling toward you, it has a centerline.
Offset is how far the mounting point is from the centerline. It's where the rim actually mounts to the axle.
Positive offset means the mounting point is farther away from the truck and centerline, so the wheel actually is mounted closer to the truck. Negative offset means the mounting point is closer to the truck, so the wheel sticks out farther from the truck. If you're facing the side of the truck, positive is pushing the wheel in toward the truck, and negative is pulling it away.
Offset is measured normally in millimeters. 25.4 mm = 1 inch. If you switch from the stock +18mm offset to a new rim with a -6mm, that's a change of 24mm, which is about 1 inch. The change is negative, going from +18 to -6, which means the tires stick out 1 inch further.