The "Dish" is the wheel mounting surface of a 12 bolt wheel. When you unbolt the 12 bolts and remove the nuts you can then remove the "Dish" of the wheel. The Picture is of a 12 bolt wheel with the nuts taken off and the "Dish" still located on the shell.
We can also machine the stock center out of the dish for an additional charge. Cutting the stock center out of the wheel aids in re-centering the wheel.