This picture shows the cationic character of an unsymmetrical bromonium
ion. The picture on the right is the same molecule rotated downward. The
two carbons that once shared a double bond are now sharing a bromine atom.
This creates an area of positive charge. However it is very important to
realize that one carbon atom is more positively charged than the other.
This will dictate the placement of substituents during a reaction. The
more substituted carbon will be more positive than the other and in this
case, it is the carbon on the left. The methyl group acts to stabilize
a carbocation so there is more positive charge at this site. You can tell
this because this carbon is not only positively charged (darker blue color),
but it has sp2 character.
This picture illustrates the reaction between a bromonium ion and a water molecule. The high electron density of the water molecule attacks the positively charged carbocation of the bromonium ion. Remember it specifically attacks the more substituted carbon because it is more positively charged. After the water molecule attaches, a base will react with the molecule and a proton from the water molecule will be lost. The hydroxyl group will therefore be added on the more substituted carbon and on the opposite side of the bromine atom (anti-addition).