"Magnesium" refers to a magnesium salt, "sodium" refers to the element sodium.
Sodium magnesium fluorosilicate is a substance used in cosmetic preparations where it acts as an abrasive (removes impurities from the surface of the body, supports mechanical tooth cleaning and/or improves shine), viscosity regulator (increases or decreases the viscosity of cosmetic products), opacifier (reduces transparency and light transmission by clouding the product) and absorbent (absorbs finely divided substances).