What does mg2co3 mean?

I believe it is Magnesium Oxide. The 2 should be subscript, denoting the presence of two atoms of magnesium. This is because magnesium has a charge of -1, versus oxygen's typical charge of +2. Two atoms of magnesium balances the compound. No, magnesium oxide would be MgO - the magnesium has an oxidation number (or charge) of +2, compared with -2 for oxygen, so one of each is needed to balance out the molecule. I don't think Mg2O exists.