Nitro group has a nitrogen atom bonded to two highly electronegative oxygen atoms. This results into a net decrease of electron density around nitrogen atom and imparts a positive δ+ charge on nitrogen.

This positive charge developed when attached to a benzene ring tends to withdraw electron density away from the ring, inducing negative resonance effect which can be demonstrated as:

The movement of positive charge shows that the ring decreases its electron density and thus is less reactive as compared to unsubstituted benzene.