Density is gross weight divided by volume. Each mode has a threshold: IATA air 167 kg/m³ (ratio 1:6000), express 200 kg/m³ (1:5000), sea LCL 1,000 kg/m³ (1 t = 1 m³). Below the threshold you pay by volume; above it, by weight.
In road groupage a convention of about 333 kg/m³ (1:3000) is common, although each operator sets its own, or prices by loading metre or per pallet instead. Knowing a product’s density tells you which mode and which tariff suit it.