This reaction is known as Acidic Cleavage of Ether. Ether when treated with strong acid like HI, the oxygen atom of ether picks the proton making a positive charge on oxygen. The Iodide conjugate base formed acts as a nucleophile and attacks on the carbon next to positive oxygen and forms Alkyl Halide. This is a type of SN² reaction. The complete reaction with missing atoms, charges and arrows is shown below,
