The width of a rim is the distance between its two edges, expressed in inches. The ET (offset or off-set) is the distance between the mounting surface and the rim's geometric center line, expressed in millimeters. The lower the ET value, the more exposed the rim is in the wheel arch. Both parameters are found in the wheel rim size.
Example:
6.00Jx15 5x100 38.00, the first numerical value indicates the width of the rim (6 inches) and the last is the ET (38 mm).