Recall that nucleophiles attack only those aryl halides that comprise strong electron-withdrawing substituents placed ortho or para to the leaving group. 

The reaction proceeds via nucleophilic aromatic substitution, involving two steps—the addition of the nucleophile and the elimination of the leaving group.

In the first step, the nucleophile adds to the electron-deficient carbon that bears the leaving group. Subsequently, the electrons delocalize over the ring carbons.

Resonance interaction with the electron-withdrawing substituent allows it to temporarily take up the electron density.

The contributing resonance forms of the intermediates are termed Meisenheimer intermediates.

Among these, the structure that bears a negative charge on the oxygen atoms of the nitro group is particularly stable.

In the second step, the nitro group liberates the electrons into the ring to eliminate the leaving group and restore aromaticity.

To summarize, in ortho and para intermediates, the electrons are delocalized over the nitro group, while in meta, they are not.

As such, ortho and para isomers successfully undergo S_NAr reactions, whereas, with meta isomers, the reaction does not occur.