SG: Starting gravity. A hydrometer is used to test the specific gravity of the liquid before fermentation. Water is 1.000.

FG: Final gravity. A hydrometer is used to test the specific gravity of the liquid at the end of fermentation. Yeast consume sugar, give off carbon dioxide and a by product is alcohol. CO2 escapes the sealed container. Theoretically the difference is a reduction in sugar and an increase in alcohol. The lower the final gravity, the dryer a wine is (less sugar or sweetness).

Hydrometer: a weighted instrument that tells the specific gravity of a liquid

Racking: siphoning the wine from one container to another. This "cleans" the wine by removing clear wine from the sediment that has settled to the bottom of the container. Usually this is done a couple of times depending on the kit. Clearing agents can be added to attract sediment and make them heavy enough to fall to the bottom.