When the vapour pressure of solution equals vapour pressure of pure solid, then the solution will freeze. When a non- volatile solid is added to the solution(here NaCl), it disturbs the dynamic equilibrium between solid and liquid and lowers the vapour pressure. Lowering of vapour pressure means that the temperature at which vapour pressure of frozen solid and liquid equals will also fall. Therefore, there is a depression in freezing point. This depression can be calculated as,

∆Tf=Kfm

Where ∆Tf is the depression in freezing point

Kf is the freezing point depression constant

m is the molality of the solution