Draw a resonance structure for nitrobenzene that demonstrates why it is a meta director. you will want to show decreased nucleophilicity at some carbon atom(s) in the benzene ring such that one carbon atom in the ring has a positive charge.

The rasonating structure of nitrobenzene is provided in attachment.

The oxygen is electronegative and attracts the electrons from benzene ring and gets negative charge on it due to which positive charge appears on ortho and para position of ring and thus the coming electrophile only attacks to meta position because the electrophile cant attack on position positions.

See explanation below

Explanation:

The nitrobenzene is meta director. This can be verified, by doing an electrophylic reaction with the nitrobenzene, and try to stabilize the molecule by resonance in orto, meta and para. Picture 1 have the resonance structure with Orto and meta.

This actually happens, because when the electrophile is in orto position, when yo do resonance to stabilize the positive charge, there is a point where the charge cannot be stabilized anymore, and remains unstable. Same thing happen with the para position. That's why in meta position the charge can be moved around the ring and the molecule is stable.

See picture for resonance.

i do not get it   at all

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.