Hello Candidate,

NO2 is a electron withdrawing group which has the charges of of negative resonance effect as well as strong negative inductive effect. This allows the NO2 group to withdraw the partial electrons towards its side making the the the attach group electron deficient and thus beares a partial positive charge.

It is one of the most activating groups in electrophilic aromatic substitution, as it produces a strong partial charges opposite nature on both the sides.

Hope it helps!!