The nitro group ($-NO_2$) is an electron-withdrawing group due to its strong $-I$ and $-R$ effects. It reduces the electron density of the benzene ring, making it deactivated towards further electrophilic substitution. The electron density at the ortho and para positions is comparatively less than that at the meta position. Hence, the electrophile (nitronium ion, $NO_2^+$) attacks at the comparatively electron-rich meta position. Therefore, the nitration of nitrobenzene requires rigorous conditions (high temperature) and yields 1,3-dinitrobenzene (meta-dinitrobenzene) as the major product.