The offset of a wheel is the distance from its hub mounting surface to the center line of the wheel. The offset is generally measured in millimeters.
Offset is usually denoted as et30 or +30. That’s positive 30 offset.
The lower the offset the more lip you will have. If you look in the diagram below, the negative offset has more lip than the positive offset. That’s why cars with lower offsets in the rear like a BMW M3 (+20 offset) have more lip than say a Mercedes that has an offset of +45.
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline. “Deep dish” wheels are typically a negative offset.