From the above diagram, the reactant which is prop-1-ene which unsymmetrical alkene, it's reacting with Hydrobromic acid (Hydrogen Bromide) which is the polar reagent. When this hydrogen bromide dissociate it yield H+ and Br-. The positive part of this polar reagent (H+) is attached to the carbon in between the double bond with highest number of hydrogen while the negative part of this polar reagent (Br-) is attached to the carbon in between the double with the lowest number of hydrogen. It's usually go with addition reactant.