Hi Promothesh,

It the process in which adding a solute to a solvent decreases the freezing point of the solvent. 

Example -  if we keep 1 glass water in freezer it will get freeze at 0⁰C  

now if i add some salt in that 1 glass of water it can freeze and become ice before 0⁰C

This is because of decrease in freezing point

Thanks for asking

Hope you got your answer