These represent various chemical compounds containing magnesium $Mg$ along with other elements:

Mg: This represents elemental magnesium.MgO: This represents magnesium oxide, a compound containing magnesium and oxygen.MgCl2: This represents magnesium chloride, a compound containing magnesium and chlorine.MgCO3: This represents magnesium carbonate, a compound containing magnesium, carbon, and oxygen.Mg3$PO4$2: This represents magnesium phosphate, a compound containing magnesium and phosphate ions.