A Lewis structure may satisfy electron counts in more than one way. Formal charge is a bookkeeping quantity used to compare how valence electrons are assigned within a candidate structure.

For an atom,

$$FC=V-N-\frac{B}{2},$$

where $V$ is the number of valence electrons in the neutral atom, $N$ is the number of nonbonding electrons shown on that atom, and $B$ is the number of bonding electrons shared in its bonds.

In carbon monoxide drawn as a triple bond with one lone pair on each atom, carbon has

$$FC_C=4-2-\frac{6}{2}=-1,$$

while oxygen has

$$FC_O=6-2-\frac{6}{2}=+1.$$

The sum of all formal charges must equal the overall charge of the molecule or ion.

Formal charge does not claim that an atom literally possesses that integer electric charge. It is a model for electron bookkeeping. When several plausible Lewis structures exist, structures with smaller formal-charge magnitudes are often favored, and negative formal charge is usually more plausible on the more electronegative atom.

These are selection heuristics, not independent physical laws. Formal charge is most useful for evaluating Lewis representations and preparing the ground for resonance, where no single electron assignment may describe the molecule adequately.