Explaining "dish" further:
On a FRONT wheel the hub is centered on the axle. And both the hub and the axle are centered relative to the rim. This is a ZERO dish wheel. The angle of the spokes coming from the right flange of the hub to the rim is the same as the angle of the spokes coming from the left flange of the hub ( i.e. the spoke angles are symetrical). BTW, the greater the spoke angle, the stronger the wheel will be, so hub flanges are generally designed to stick out as far as the fork width will allow.
On a REAR wheel there has to be room on the right side of the hub for the gear cluster--the more gears, the more room that is needed. So, the right hub flange doesn't extend as far outward as the left hub flange. Thus, the spoke angle of the spokes coming from the right side of the hub toward the rim is less than the angle of the spokes coming from the left side of the hub toward the rim. This asymetrical spoke angle is called "DISH".
The less a spoke is angled, the tighter it has to be to hold the rim in place. So the drive-side spokes end up at a higher tension than the non-drive-side spokes.
Clear as mud?