The relative weight or density of a solid or fluid expressed by the ratio of its weight to that of an equal volume of a substance taken as a standard, water in the case of liquids and solids, air for gases. e.g. Water = 1.

Other examples:
Whisky = 0.90
Turpentine = 0.86
Kerosene = 0.80
Fruit Juice = 1.34
Phosphoric Acid = 1.8 - 1.9
Bromine Trifluoride = 2.49
Aviation Fuel = 0.63 - 0.79

The higher the SG, the smaller the tank will be, all things being equal.