Resonance averages bond character across equivalent positions. In $CO3^{2-}$, three equivalent C-O bonds exist with bond order = $\frac{1 double + 2 single}{3}$ = 4/3 = 1.33. All three bonds have identical length (intermediate between C-O single and C=O double). In benzene, each C-C bond has order 1.5 (alternating single-double averages out). In NO3-, three equivalent N-O bonds with bond order 4/3. In $SO4^{2-}$, if we consider expanded octet resonance structures, bond order of each S-O bond can be computed similarly. Resonance-stabilised structures are more stable than any individual Lewis structure and have lower energy. The more equivalent resonance structures, the greater the stabilisation.