A hydrometer is used in beer & wine making applications to determine the amount of dissolved sugars in the wort or must. Recording & tracking the standard gravity of any fermenting beverage is crucial to understanding the progress of the fermentation. During this time, yeast (and other microorganisms depending on your beverage) are consuming dissolved sugars and releasing by products such as alcohol and carbon dioxide. Using a hydrometer to observe the gravity before fermentation begins will allow you to determine the progress and rate of that sugar being removed at a later sample.
When fermentation has completed, the gravity of the sample will no longer change between times of sample. Recording this final gravity (FG) and comparing to the original gravity (OG) allows a brewer to use an ABV CALCULATOR to determine the alcohol by volume of the beverage.