The measure of the density of a liquid specifically grams of sugar per liter of water. A gravity of 1.040 means 40 grams of sugar per liter. "Specific" refers to "this sample right now".  This will show you the amount of potential alcohol you may end up having if your yeast ferments most of the sugar.  That is why you want to take a reading before your introduce your yeast.  If you do not do this you will not know how much alcohol you have in your finished product.