Unit weight is the weight of a material per unit of volume, usually in pounds per cubic foot.
For compacted hot mix the common default is 145 pounds per cubic foot. That's why the usual tonnage formula divides depth by 12 and multiplies by 145. The real figure for your mix is on the plant's mix design and usually lands somewhere around 140 to 150. Compacted crushed stone base often runs around 2 tons per cubic yard, which is about 148 pounds per cubic foot, and normal-weight concrete is about 145 to 150.
The gap between the default and the real number is small per cubic foot and big per job. A 3 percent difference on a 400-ton lot is 12 tons of mix. Ask the plant for the unit weight of the mix you're ordering and put it in the estimate.