The molal depression constant depends upon

Depression in freezing point is denoted by ΔT and depression in freezing point is directly proportional to the molality of solvent.
ΔT∝M
Here M is molarity of solvent
For removing proportional signs we need a constant and this constant is a molal depression constant.
ΔT=K_f.M
Molal depression constant is represented by K_f that depends upon quality of solvent and molality is the nature of solvent