It is a reading your get from a hydrometer and it is very important. You can get a hydrometer from your local brew shop or from one of our vendors here, they are only about $6 but buy a couple cuz I will guarantee you when you only have one, you will break it, when you have more than one, you don't seem to break it.
When you start a wine, you take a hydrometer reading to know how much sugar you have in your must, this will give you a good idea on how much alcohol you will have in a finished wine. A hydrometer reading will also let you know if you have a stuck fermentation. We have a tutorial section here that will explain and show you haw to read a hydrometer. SG means starting gravity and FG means final gravity