Shapton Lapping Powder is meant to help you lap your (Shapton) sharpening stones. The bottle is filled with medium silicon carbide grains. You use these grains in combination with, for instance, the Shapton Naoru Lapping System or a glass lapping plate.
The medium lapping powder is used to lap sharpening stones with a grain size ranging from 1000 to 5000. If you are planning to lap a sharpening stone with grain size of less than 1000, use the coarse lapping powder. If you are planning to lap a sharpening stone with grain size of less than 5000, use the medium lapping powder. Properly clean the lapping plate after use to make sure you grains are left behind.