The SG of something is a reference to it’s density relative to water.
The SG of water is 1.
This means water has 1kg of mass per litre.
Gold has an SG of 19.32.
This mean that it weighs 19.32 kg per litre.
Thus it will sink in water.
Ice has an SG of 0.92.
This means it weighs 0.92kg per litre.
Thus it floats in water.
Many SG tables talk in kg/cu.m or said as kilograms per cubic metre.
There are 1000litres in a cubic metre.
Cubic metre = cu.m = m^3 = m³
This table at Engineering Toolbox give you some more SG’s of different substances.
The first minute and a half of the video below also gives a good description of SG.