The reaction is

$C{H}_2=CH-C{H}_3+HBr\to C{H}_3-CHBr-C{H}_3$

During addition reactions, the attacking reagent adds on to the substrate molecule without elimination. In this process, a triple bond may be converted into double bond or single bonds and a double bond is converted into single bonds. In the example we can see alkenes are electron rich and Hydrogen ion will attack on the double bond. This is called as electrophilic addition reaction.