-Specific gravity , symbolized sp gr, refers to the ratio of the density of solid or liquid to the density of water at 4 degree celcius.
-it is a way of measuring density. It is dimensionless, has no units. 

SG is the density of the material divided by the density of water. 

for example 
density Lead 11340 kg/m³ 
density of fresh water at 20C = 998 kg/m³ 
therefore SG of lead = 11340 kg/m³ / 998 kg/m³ = 11.36

Good Luck ~ Ganbatte