Backspacing is also important to know, in addition to the offset. They are not the same dimension, but one will affect the other. With a 15x6, you would want something around a 3" backspace with a 0 MM offset.
They're a different way of measuring the same thing. Offset is the "offset" of the mounting surface from the center of the wheel, and backspace is the measurement from the mounting face of the wheel to the back edge of the wheel. That's why your 15x6" wheel example with a 3" backspace is 0 mm offset. You need to know only one or the other, not both.
I said offset but always use backspace, as I find it easier to work with. Most of the old car stuff refers to backspace, and stuff for the more modern rice-burners or positive-offset stuff (no or little wheel lip) refers to offset.
