The mechanism is an Electrophilic Addition. Explanation of the drawing: Draw a curly arrow from the but-1-ene double bond to the H and a curly arrow from the HBr bond to Br; forming a secondary carbocation and bromide. Lone pair on bromide attacks postive carbon and bond forms. Three isomers: two enantiomers/ optical isomers of 2-bromobutane (major product); 1-bromobutane formed as minor product via primary carbocation.