What does the symbol MgSO3 means?

Answer 1

Likely it is;

Magnesium sulfite

Mg(2+) and SO3(2-)

but SO3 can also be sulfur trioxide which also has an oxidation number of 2-

then;

Magnesium trioxide would be the name, but this is much less likely

Answer 2

The symbol MgSO3 represents the compound magnesium sulfite, which consists of one magnesium ion (Mg^2+) and one sulfite ion (SO3^2-). Magnesium sulfite is typically used in food preservation and as a reducing agent in organic synthesis.