from wikipedia - Gravity, in the context of fermenting alcoholic beverages, refers to the specific gravity, or relative density compared to water, of the wort or must at various stages in the fermentation. The concept is used in brewing and wine making industry. Specific gravity is measured by a hydrometer, refractometer, pycnometer or oscillating U-tube electronic meter.
The density of a wort is largely dependent on the sugar content of the wort. During alcohol fermentation, yeast converts sugars into carbon dioxide and alcohol. The decline in the sugar content and the presence of ethanol (which is appreciably less dense than water) drop the density of the wort. The percentage of alcohol can be calculated from the difference between the original gravity (abbreviated OG) of the wort and the current specific gravity (abbreviated SG) of wort. By monitoring the decline in SG over time the brewer obtains information about the health and progress of the fermentation and determines that it is complete when gravity stops declining. If the fermentation is finished, the specific gravity is called the final gravity (abbreviated FG). For example, for a typical strength beer, OG could be 1.050 and FG could be 1.010.