Question

what do you mean by relative density of an object?

Answer 1

Relative density, or specific gravity, is the ratio of the density of a substance to the densityof a given reference material.
If a substance's relative density is less than one then it is less dense than the reference; if greater than 1 then it is denser than the reference.

Answer 2

Relative density of a substance (object) the ratio of the density of substance to the density of water.

Or
Relative density of a substance is the ratio of mass of any volume of substance to the mass of an equal volume of water.