This is a general explanaton, since you have not included details of your experiment. But a valid and useful one for you.
From your knowledge about colligative properties you know that the freezing point depression is caused by the presence of a solute:
ΔT = kf × m × i
In this case the solute is sodium chloride (NaCl) which when dissolved will cause that the freezing point ot the water will be lower. That will help to have more liquid water at lower temperature. Since, the liquid water has a better heat transference than solid ice, the result is the mixture will be more efficient to cool the surroundings.