Aromatic ether to phenol reagents
The reaction is demethylation of methoxybenzene. You can do it by using an acid which has a nucleophilic counter ion.  



It is not actually obvious why the iodine nucleophile attacks the methyl group and not inside benzene ring. The other option would be:
 

This reaction doesn't happen because otherwise the iodine ion would appear inside the benzene ring and resulting product would be in a very strange conformation (high energy).
More information:
 Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. p. 434, 589.     

As it was correctly stated in the other answer there are also other reagents which can be used to cleave the methyl ether. $\ce{BBr3}$ is a Lewis acid which also can be used in this reaction. A relatively new paper describes a computationally calculated bimolecular mechanism for this reaction.