offset can also be referred to as back spacing.
offset can be either postive or negative.
positive offset is moving the mounting flange from the center toward the *face of the rim
negative offset is moving the mounting flange from the center toward the backside of the rim
negative offset gives that deep dish look very common on old muscle cars like the 67-69 camaro
face = the side you see when the wheel is mounted on the car
backside = the side of the rim that you can't see when mounted closest to the brakes
For example. take a generic 15" diameter rim that is 6" inches wide.
If the mounting flange was exactly in the center of the rims width the offset/back spacing would be 0 (zero)
now relocate the flange out toward the face of the rim a few milimeters and you've got yourself a rim that's 15" in diameter 6" wide with a +30mm offset.