In air freight, the rate is calculated per unit of weight (e.g. $1.50/kg).
However very non-dense cargo takes up significant room without weighing much. (Think if you were to ship inflated balloons–they weigh almost nothing yet take up valuable room on the plane.)
For this reason, airlines quote by the greater of weight or measure. Weight being actual weight, and measure being the “dimension” (or “volume”) weight. For international shipments, dimension weight is usually calculated by dividing the total cubic inches of your cargo by 166. If this number is greater than the actual weight, then your cargo will be measured as dimension weight. If it is less, then your cargo is dense enough to be measured by its actual weight.