In the presence of peroxide, HBr addition follows the anti-Markovnikov rule (Kharasch effect).
The reaction proceeds via a free radical mechanism:
$$\text{ROOR} \xrightarrow{\Delta} 2\text{RO}^{\bullet}$$
$$\text{RO}^{\bullet} + \text{HBr} \rightarrow \text{ROH} + \text{Br}^{\bullet}$$
$$\text{Br}^{\bullet} + \text{CH}_3\text{CH}=\text{CH}_2 \rightarrow \text{CH}_3\text{CH}^{\bullet}\text{CH}_2\text{Br}$$
The $\text{Br}^{\bullet}$ adds to the terminal carbon, forming the more stable secondary radical, giving 1-bromopropane.