Let's look at pH and the two chemicals in question. pH is a measure of the acidity (proton concentration) of a solution. Acidic solutions are lower in pH and basic solutions are higher in pH. Ammonium hydroxide is NH4OH. It can be thought of as a solution of ammonia (NH3) in water (H2O). Ammonia is a weak base. As a result, ammonium hydroxide is a basic a chemical and will have a pH greater than 7. Ammonium chlorde (NH4Cl) can be thought of as a mixture of ammonia and HCl. HCl is a very stron acid, so NH4Cl is acidic and a mild source of protons. So if NH4Cl is added to NH4OH, the pH of the solution will go from a higher number (basic) to a lower number (acidic). This accounts for the observation.