The term "relative density" means the same as "specific gravity." It is a measure of how dense one thing is when compared to a given reference. The reference is usually water. With respect to water, the relative density of mercury is 13.56 because it is 13.56 times as dense as water.
The absolute density (how much a given volume of the substance weighs) of mercury is 13593 kilograms per cubic meter. This can also be stated as 13.56 grams per cubic centimeter (by definition, the density of a substance in kg/m^3 is 1000 times the density in g/cm^3).