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!