A measurement made using a Hydrometer? in making wine, beer, or mead. It basically tells how much sugar is in the must.

Percent Alcohol from Specific Gravity

By measuring the SpecificGravity before adding the yeast and then again after fermentation is complete, the difference can be used to calculate the PercentAlcohol?.

Amount of Sugar to Add

By measuring the SpecificGravity before adding any sugars, you can both determine how much natural sugar is contained in the ingredients and also calculate how much additional sugar to add to obtain the desired PercentAlcohol?.

When to Move from Primary to Secondary Fermentation to Racking

Taking measurements periodically during the fermentation phases can signal when to move to the next stage. Many people recommend the PrimaryFermentation? is over when SpecificGravity falls below 1.010. Many kits I've come across use SpecificGravity measurements to indicate that fermentation is over (i.e. below 1.005 for sweet wines, below 0.995 for dry wines, etc.)

More information at WikiPedia:Specific_Gravity.