Please explain reactions involving N-Bromo succinimide.and why is it used.

NBS is  generally used for allylic/benzylic  bromination...

eg.

Standard conditions for using NBS in allylic and/or benzylic bromination involves refluxing a solution of NBS in anhydrous CCl₄ with a radical initiator, usually azo-bis-isobutyronitrile (AIBN), benzoyl peroxide, irradiation, or both to effect radical initiation. This is also called the Wohl-Ziegler reaction.

The carbon tetrachloride must be maintained anhydrous throughout the reaction, as the presence of water may likely hydrolyze the desired product.Barium carbonate is often added to maintain anhydrous and acid-free conditions.