In layman's terms, the test measures the viscosity of the melted plastic, which is directly related to its molecular weight. The test is performed on a specialized apparatus called an extrusion plastometer or less formally, a melt indexer. The test is described in ASTM Standard D-1238. Specific gravity is the ratio of the weight of a given volume of material to the weight of an equal volume of water. Density in PE tells a great deal about the physical properties that the material exhibits.
More accurate testing is more involved.
Bulk Density Bulk density is the weight of a given volume of material, including the void spaces inherent in it. This is an important quality in bulk material handling applications, such as pneumatic conveying. It is also very important in rotational molding powders, since bulk density directly indicates the fluffiness of a powder. When pulverized, the particles of PE tend to form hairs. Under a microscope, they can look more like dust-bunnies than like grains of sand.