The Final Gravity is the Specific Gravity of the fermented beer. It will always be less than the original gravity because during fermentation heavy sugars are converted to lighter carbon dioxide and alcohol. The gravity is reduced both by the reduced sugar content, and because alcohol is lighter than water. The Specific Gravity of most beers will start in the range of 1.040 to 1.055. The Final Gravity will usually range from 1.006 to 1.018. The higher the Final Gravity the more body the beer will have.
The weight of a beer after fermentation.
The density of sugar to water after fermentation.
The Specific Gravity of the Must, measured after fermentation has stopped.