????

[Kimberly 1H]

"Broadly speaking, the wavefunction that describes the resonance structure is a more accurate description of the electronic structure of the molecule than the wavefunction for any single structure alone, and the more accurate the wavefunction, the lower the corresponding energy."

I have no idea what this means.

[Harshita Talkad 4L]

The first part is explaining how a combination of all the resonance structures is a more accurate representation of the bond lengths than each individual resonance structure on its own.

[Matthew Mar 1J]

Each type of bond has a different length (single bonds are longer than double bonds, etc...), so when we draw a Lewis structure that has both single and double bonds that can be switched around or interchanged (take the classroom example of NO3-, where there are two single bonds and one double bond, but the double bond could be between any of the three oxygens), all the different possible configurations blend together in the actual molecule so that the bond lengths average out and make the molecule more stable.