"The specific gravity of a substance is a comparison of its density to that of water. Imagine a gallon bottle filled with water, a second filled with feathers, a third filled with lead weights. There are equal volumes of material present, but the bottle with the feathers will weigh less than that containing water; the bottle with lead weights will weigh the most. In order of increasing specific gravity, these materials would be: feathers, water, lead. Specific gravity can be measured precisely, or estimated by a comparison, as above. To compare the specific gravity of any two minerals, simply hold a sample of one in your hand and "heft it," i.e., get a feeling for its weight. Then heft a sample of the other that is approximately the same size. If there is a great difference in specific gravity, you will detect it easily. It is often sufficient to note whether a mineral's specific gravity is significantly higher or lower than that of other minerals. Heft each of the specimens in your mineral set. Which ones have a high specific gravity? A low specific gravity?"
How does it relate to urine? You check specific gravity of urine to detect normal kidney clearance of waste, electrolytes, and other substances.
If someone has dibetes insipidus, a disease in which there is dec release of anti-diuretic hormone (ADH), the kidneys cannot conserve water in the blodd volume, so more water is excreted, and the urine is like water, and will have a lower specific gravity.
If someone has pyelonephritis, the semi-permeable membranes in the kidney tubules are inflammed, and may allow proteins to escape into the urine. Protein is not normally there, so there is a higher spefici gravity.
Anytime there is more or less of a mineral or compaund in the urine, it will affect specific gravity. We know the normal specific gravity of urine, checking specific gravity is a fast way of knowing you better look for something goin on there.
Hope that made sense, and I did it w/o my ref books handy, so you better double check my analogies if you intend to use them! LOL!