Specific gravity is used to measure the amount of sugars dissolved in the wort and the remaining sugars and alcohol after fermentation. Yeast consume sugars and produce alcohol, which is lighter than water. Specific gravity is therefore lower after fermentation. Given the original gravity and the final gravity, the brewer can therefore calculate the alcohol percentage and attenuation of the finished beer. Since specific gravity is the ratio of the density of a liquid to the density of water; pure water therefore has a specific gravity of 1.
The gravity after fermentation is referred to as the final gravity or FG.