Definition: Similar to track width, the wheel offset has more to do with the distance of the wheels from the center of the RC rather than the distance between the wheels.
If the hub has a positive offset, it means that the center of the rim is further from the hub and further from the centerline of the vehicle giving you better handling. If the hub has a negative offset, the center of the rim is much closer to the hub and closer to the centerline of the vehicle, which is a stock setting on most RCs.