When you are talking about the mass by unit volume, it is called as density.
For eg. Density of water = 1000 kg m-3
Density of any other substance = x
Depending upon what the value of 'x' is, the other substance may sink / float.
But when you are 'comparing' the density of value of object x to the density of water, it is reffered to as relative density.
For eg. Density of any substance = 1000 kg m-3
But, if Density of water = 1 kg m-3,
Then acc. to the formula,
Relative Density = DENSITY OF THE OBJECT/DENSITY OF WATER
= 1000/1 = 1000
Since it is more than the 1, obviously it will sink.
This is the concept.
Hope it helps.
Cheers!