For questions about formal charge in any molecule. Do not confuse with [oxidation-state].
Formal charge is a bookkeeping tool in chemistry. A formal charge (FC) is the hypothetical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.
When determining the best Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge on each of the atoms is as close to zero as possible.
Adapted from Wikipedia