A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent .What it do mean .
Actually, the vapour pressure is directly proportional to temperature so that decrease in vapour pressure will decrease temperature.
Now, you need to understand that, when a solution boils the vaporising component is its solvent and not the solute, similarly if a solution cools then the freezing component will be the solvent and not the solute and so it is said that if vapour pressure becomes equal to the vapour pressure of pure solid solvent, then solution will be solidified and hence solution will freeze.
Now, you need to understand that, when a solution boils the vaporising component is its solvent and not the solute, similarly if a solution cools then the freezing component will be the solvent and not the solute and so it is said that if vapour pressure becomes equal to the vapour pressure of pure solid solvent, then solution will be solidified and hence solution will freeze.