Different materials have different density. The density can be described as the mass divided by the volume, for a homogeneous object. Water have a density of 1kg/L (1/cm3) and is often used as reference. It also makes it easy to test if the density of material that have lower density than water - as they float.
When designing lightweight structures the lower density materials can be utilized to make lighter structures. A composite truss of the same size and shape as an aluminum one is therefore significantly lighter.