Hydrohalogenation reaction is one in which a hydrohalic acid (HBr,HI or HCl) is added electrophilcally to an alkene to for its corresponding haloalkane or with an alkyne to form the corresponding haloalkene.
Regioselectivity means where the functional group will react.
Regio means region or position and selectivity means preference given by reagent to a particular site (atom or group of atom) in substrate molecule while doing reaction with it.
Sometimes it may happen that a substrate molecule may offer two reacting sites for the reagent to attack, but the reagent chooses the one which gives most stable product. This is regioselectivity.
For Ex:- when secondary alcohols undergo elimination reaction in presence of conc. sulfuric acid, there are two possible sites from where hydrogen can be abstracted, but the reaction chooses the hydrogen which after elimination which gives more stable alkene.