SG usually stands for specific gravity which is what you measure any time you measure. Brewers will say OG for Original Gravity and FG for Final Gravity. What you are measuring is the amount of sugar that is in the liquid you are measuring. Water will basically have an SG of 1.000. You measure the OG before you ferment and will get a number somewhere between 1.030 and 1.120. The lower the number the less sugar that is in solution. The yeast eat those sugars to produce alcohol, so if we know how much they ate, we know how much alcohol there is. You take another SG reading after fermentation is complete(your FG) and that will tell you how much sugar is left, usually between 1.000 and 1.030. Knowing the difference in these two numbers will tell you how much alcohol is in your beer/wine/mead/etc. Some people use a really basic formula where you take the difference and multiply by 131. For example, if you had an OG of 1.045 and an FG of 1.010: 1.045-1.010= 0.035*131= 4.585 so your beer is 4.5%abv. There are better calculators than that online such as the one above that gives you an answer of 4.7% with the same numbers.