2 NH4ICl + Zn0 ZnIICl2 + H20 + 2 NH3I

This is an oxidation-reduction (redox) reaction:

Zn0 - 2 e- ZnII (oxidation)

2 HI + 2 e- 2 H0 (reduction)

Zn is a reducing agent, NH4Cl is an oxidizing agent.