What does it mean by 'offset' when buying some alloys?

Rim offset, more accurately referred to as wheel offset, is the distance from the hub mounting surface to the centerline of the wheel. If this measurement isn't suited to the specific vehicle, the vehicle won't handle as it should. Wheel offset measurement is defined in three different ways. Zero offset occurs when the hub mounting surface is completely even with the centerline of the wheel. Positive offset means that the hub mounting surface is toward the front of the wheel, while negative offset means the surface is toward the back or brake side of the wheel.