Usually formal charges are not found in Lewis structures, but some molecules require them. Formal charges do not always occur in molecules with resonance structures, as is the most famous case of benzene. The resonance stabilization of benzene and its analogs is sometimes called 'aromatic' stabilization. Sometimes the formal charges can help one decide between the relative importance of non-equivalent resonance structures. The nitrate anion is symmetric, so either an incomplete octet, or three equivalent resonance structures can predict this.
Resonance is the preferred explanation.
This means we have to think a little more carefully about the orbitals that are involved in chemical bonding. The orbitals that electrons are in when they exist in molecules are actually different from even the most 'hybridized' or perturbed atomic orbitals. How molecular orbitals for stems from how waves interefer with one another and the concept of Phase. Adding orbitals 'in phase' or 'out of phase' can make two different shapes, usually one that is 'bonding' and one that is 'antibonding'. Antibonding orbitals have a new node between the atoms and perpendicular to the bond axis.