Complex sentence: Deactivating groups such as other nitro groups have an electron pair withdrawing effect which deactivates the reaction (creating difficulty in the formation of poly-nitro products) and directs the electrophilic nitronium ion to attack the aromatic meta position.
Split into simpler sentences: Deactivating groups such as other nitro groups have an electron-withdrawing effect. Such groups deactivate (slow) the reaction and directs the electrophilic nitronium ion to attack the aromatic meta position.