$NBS(N$-bromosuccinimide) is used for allylic bromination. The given compound in equation undergoes restrictive allylic bromination due to the low concentration of $Br _{2}$ because $N$-bromosuccinimide $(NBS)$ is present with a trace amount of acid (HBr), $HBr$ will react with NBS to give succinimide and $Br _{2}$.
