This page looks at the reaction of the benzene ring in phenylamine (aniline) with bromine water. It explains why the amino group activates the ring.
Activation of the ring
The -NH2 group attached to the benzene ring in phenylamine has the effect of making the ring much more reactive than it would otherwise be. This is exactly the same as the effect of the -OH group in phenol if you have already come across that. For example, phenylamine will react with an aqueous solution of bromine (bromine water) in the cold and in the absence of any catalyst. Unactivated rings will only react with bromine in the presence of a catalyst.
The lone pair on the nitrogen touches the delocalized ring electrons . . .
. . . and becomes delocalized with them:
A benzene ring undergoes substitution reactions in which the ring electrons are attacked by positive ions or the slightly positive parts of molecules. In other words, it undergoes electrophilic substitution. If you increase the electron density around the ring by involving extra electrons from the -NH2 group, it becomes even more attractive to incoming electrophiles. That's what happens in phenylamine.
The directing effect of the -NH2 group
The -NH2 group has more activating effect on some positions around the ring than others (for reasons which go beyond UK A level). That means that incoming groups will go into some positions much faster than they will into others. The net effect of this is that the -NH2 group has a 2,4-directing effect. That means that incoming groups will tend to go into the 2- position (next door to the -NH2 group) or the 4- position (opposite the -NH2 group). You will get hardly any of the 3- isomer formed - it is produced too slowly.
The reaction with bromine water
If bromine water is added to phenylamine, the bromine water is decolourized and a white precipitate is formed. This is exactly like the reaction which happens with phenol. The precipitate is 2,4,6-tribromophenylamine.
Notice the multiple substitution around the ring - into all the activated positions. (The 6- position is, of course, just the same as the 2- position. Both are next door to the -NH2 group.)
Jim Clark (Chemguide.co.uk)