Specific volume is quite simply, the reciprocal of density.
More accurately, it is the volume occupied by unit mass.
The SI unit is m3/kg
Although it might seem a bit weird to use this parameter when density is already perfectly familiar, the specific volume is useful when talking about things which have very low densities; gases, for example.
It is one of those engineering measurements which are used to compare different materials. Compare Specific heat capacity , specific impulse and, I guess, specific gravity