Specific Gravity (SG) is the measure of dissolved sugars in your must or wort. Using a hydrometer and measuring the SG before and after fermentation you can determine the alcohol content of your batch.
What is Original Extract?
Original extract content is the amount of liquefied malt material present in the unfermented wort, and is primarily comprised of malt sugar, protein, vitamins, and natural aromas. The fermentation breaks 1/3 of the extract into alcohol and another 1/3 into carbonation. The final 1/3 remains as extract.