AlCl3 (Aluminium chloride) is a strong Lewis acid, commonly used as a catalyst for halogenation (especially chlorination)of the aromatic group, it also leads to the creation of carbonation which is used in an electrophilic substitution reaction.
In the halogenation reaction, being a lewis acid coordinated to chlorine atom, thus break the bond
Hence it will increase the electrophilicity of its binding partner making the binding partner more reactive.
The above points and diagram clearly explains the action of act as catalyst.