If you don't understand it, then I did a poor job of writing it.
Borax has a number of uses in photography, but if you buy developers ready-mixed or in powder form ready to mix, you may never handle borax as such. If that occasion arises, you may wonder if the borax you buy at the supermarket laundry section is suitable.
There are two problems one can encounter when measuring out a precise weight of borax powder. One is impurity of various kinds, and the other is variation in weight due to water of crystallization. Borax will crystallize with either 5 molecules of water or 10 attached to a molecule of sodium tetraborate. The dry powder is likely to be amixture of the two forms, and the proportions may change over time due to humidity in the air when you do the weighing. High priced analytical grade borax may be guaranteed to be nothing but sodium tetraborate and water, but the easiest way to assure that you are measuring out the intended amount of the borate is to make a solution that is saturated at a certain temperature. Then at any higher temperature, the weight of sodium tetraborate per unit weight of solution will be constant and can be expressed as either the decahydrate or the pentrehydrate (different numbers, of course). For most of our uses, the weight of borax per unit volume of solution will be constant above the saturation temperature. At 20 C, a saturated solution of borax is 4.71%.
As to purity, the process of crystallizing borax from a 100 C solution at or near saturation will leave most of the soluble impurities, including colloidally suspended particles, in the solution at 20 C. That solution is then discarded, The process may be repeated, each time with a loss of 47.1 grams per liter of discarded solution. When you are satisfied that the remaining borax is sufficiently pure, it my be left in the bottom of a container of pure water which will become a saturated solution whose borax content will be known.
Is it worth the trouble? Probably not if all you want borax for is to make a batch of D-76 or the like. I and I'm sure many others have used 20 Mule Team borax from the grocery store for many years.