Aromaticity is a special kind of resonance delocalization, one we’ve actually already seen. Delocalization of electrons always makes molecules more stable (compared to imaginary molecules where the electrons cannot spread out). If this delocalization takes place in a flat ring of n carbons, and the number of electrons involved is 4n + 2 (i.e., 2, 6, 10, 14, etc.), then the system is called aromatic.