Step$-1:$ Peroxide undergo fission to give free radicals $R - O - O - R \to 2 - R - \dot O$

Step$-2 :$ $HBr$ combines with free radical to form bromine free radical $R - \dot O + HBr \to R - OH + B\dot r$

Step$-3 :$ $B\dot r$ attacks the double bond of the alkene to form a more stable free radical

Step$-4 :$ More stable free radical attacks the $HBr$
$C{H_3} - \dot CH - C{H_2} - Br + HBr \to \mathop {C{H_3}C{H_2}C{H_2}Br}\limits_{{\rm{n - propyl bromide}}} + B\dot r$

Step$-5 :$ $B\dot r + B\dot r \to B{r_2}$

$C{H_3}CH = C{H_2} + Br \to \mathop {C{H_3} - CH - C{H_2}Br}\limits_{(more\,stable)} $

$C{H_3}CH = C{H_2} + Br \to \mathop {\begin{array}{*{20}{c}}
  {Br} \\ 
  | \\ 
  {C{H_3} - CH - C{H_2}} 
\end{array}}\limits_{(less\,stable)} $